Dumping waste
One of the biggest problems facing modern society is a lack of places to dump waste. This not only applies to traditional waste such as plastics, but also nuclear waste from nuclear power plants and even carbon dioxide from internal combustion engines (which is “waste” of a sort). If you had a magic time portal to the Cretaceous Period, even if it was to a hole thousands of meters beneath the Earth’s surface, one of the best things you can use it for is to get rid of all this waste. Plastic gets crushed back into oil due to all the heat and pressure beneath the Earth’s surface, nuclear waste has millions of years to decay and no longer become radioactive, and most importantly of all you get this all out of modern Earth’s biosphere.
Of course eventually you’re going to run into problems due to converting all the organic compounds of your home Earth into trash and shipping them into the late Cretaceous, but then you have an entire biosphere’s worth of organic matter that is easily exploitable. Just mulch the dinosaurs and turn them into plastic Coke bottles. Yes, the whole thing is horribly irresponsible, enables extremely unsustainable modes of life (and is basically no different from modern “dump everything into the ocean” lines of thought), and ends up destroying an entire unique biosphere just to get the plastic to make the latest iPod, but it’s theoretically feasible.
Other commonly used reasons for going into the Cretaceous are difficult to justify. The flora and fauna will be of very little use to the present day beyond biomedical research. Bringing any species back to home Earth to repopulate the ecosystem will likely result in invasive species at best (darn tyrannosaurs, get out of my garbage!) or the introduction will fail due to lack of supporting elements (e.g., gut microbes, pollinators [which did exist during the Cretaceous], symbionts, etc.). Farming will be very difficult. It’s hard to say if modern crops will do well in the Cretaceous, but given the fact that most crop plants are angiosperms and therefore dependent on pollinators is unlikely they will do well. What pollinators do exist would probably not recognize them as viable.
At the same time, the only native life that would be able to be easily harvested is meat. Exactly what the plant life looks like is heavily dependent on when you go in the Cretaceous. Early Cretaceous floras were dominated by gymnosperms (i.e., conifers, cycads) and looked very Jurassic, but by the Campanian (84-72 Ma), if not a little earlier, forests were primarily angiosperm dominated and looked essentially modern. However, even by the Campanian most the plants that were present were not those that coevolved with vertebrates to produce easily edible fruits (which may have been driven by things like the evolution of primates much later) and so you wouldn't have fruit-bearing trees or most edible grains. The closest you get in the modern day is the sago cycad, which is very labor-intensive to process (specifically, removing toxins). So it’s unlikely that you could farm native plants. By contrast you could harvest meat pretty easily, and the edibility of meat has remained constant across the years. Stripping the seas of fish to feed people back in Home Earth is a possibility (aided by the fact that sea levels were at their highest during this time and much of the ocean area was shallow and good for fish productivity), though at the same time the late Cretaceous is known for having so many large predators in the oceans it has been referred to as “Hell’s Aquarium”. Large predators would make fishing difficult. Especially if you have to process the catch on land, which would draw in carnivores for kilometers.
Setting up settlements in the Cretaceous would be very cost inefficient beyond a last-ditch Hail Mary effort. You have all the problems outlined above, except now you also have to take care of people on site instead of shipping everything back to home Earth. You have to deal with diseases, parasites, and bacteria you have no immunity to. Cross species diseases might be rare since the largest wildlife are not mammalian, but then again avian-to-human transition is known. It’s unclear how well you could survive without doing something like New Zealand and destroying the entire native ecosystem and replace it with an artificial Europe-Asia-North America based one. Australia and New Zealand are good examples of what you might encounter, their ecosystems are distinctly Gondwanan, and so when European farmers and ranchers settled there they encountered huge problems due to the lack of ungulates or dung beetles that ate the poop of large placental ungulates. And of course you have to deal with the local wildlife, herbivores that bring all the problems people in Africa have to deal with megafauna (e.g., elephants eating crops), and of course the carnivores, who on the one hand probably wouldn’t recognize humans and their livestock as prey but on the other hand wouldn’t be afraid to walk straight into town and start making trouble.
As @AlexP mentioned, fossil fuels are going to be very expensive to remove the Cretaceous and bring back to home Earth. Most of the oil reservoirs should be there, the most prominent oil deposits known today are from the Cretaceous but depending on when you go in the Cretaceous the oil has already had millions of years to form. The difference in time between a 66 million year old late Cretaceous ecosystem with Tyrannosaurus and a 125 million year old one with Utahraptor and Iguanodon is the same time gap as between Tyrannosaurus and the present day. The time gap between Tyranosaurus and Styracosaurus is the same kind of time gap between Homo sapiens and Australopithecus. Additionally, very few petroleum workers are going to want to work in an environment that has large theropods prowling around on land and mosasaurs in the water.
