**When overpopulation is such that it's cheaper and more acceptable to send a person on a one way ticket than to send a robot.**

OK, so that's harsh, but you don't need to bring probes back, coming back more than doubles the cost and complexity of the mission.

The only real reason to send humans on a recon mission is to drive the plot or if intelligent life is found. Anything else can be done remotely and better by robots. A bot can sit in one place quite happily for years just pumping back data and images, never moaning about potatoes. There's never going to be a panic when you realise the bot is still alive and you have to go back and get it.

Obligatory xkcd: https://xkcd.com/695/