(Beforehand I'll excuse myself for my english. If you didn't need to know that you can just ignore it)
There's a network of root-like life forms in the top layers of my planet. They use some kind of synthetic bacteria to get energy out of chemical compounds. Also they use the warmth of the planet to grow... The outer forks of these systems are thinner than human hair. They only start growing when they detect materials the "mother system" can use to mine it and transport it. If a big fork isn't needed any longer it dies and gets eaten by smaller ones.
The biggest roots serve as storage and also function as reliable transport routes through harsh terrains like a desert or even an ocean to link two big systems together.
When two big systems "find" each other, the two biggest roots start winding around each other to share as much surface as possible over which they can pass resources. Eventually they will build towers like that. These "root towers" are the most important part to me because their iconic design should set the background theme of my world. Many animal-like creatures will use them in many different ways. The problem is that I want them to be very thick and up to higher than a mile. I don't worry as much about how the towers can get this high because there's enough carbon and network with trillions of roots will have enough energy left. I worry about why the roots would evolve this way. Now...
Why would they grow like that?
I only have a few explanations and they can't stand on their own.
I'd explain that they grow vertically because the root systems would use pressure and vacuum supported by gravity to send and receive resources.
Another attempt is that they would have to grow way up to reach gases and sunlight which they need to build certain molecules. The system would use these molecules to get a picture of where their resources should be sent to. The higher the concentration of these molecules in a root, the more of the requested resources it gets.
This sounds silly to me, but the root system could use these towers to protect a big (and very high) store of reserves from creatures that eat roots underground but not on the surface. Not energy efficient, but hey...
What I want
I want one single solid explanation to why the towers would grow this big. The readers shouldn't ask themselves much more about the towers and they should just accept that they are a scientifically logical thing to exist.
How far off do you think I am?
Do you have a better explanation than any of mine?
Do you have any other advice for me?
Thank you in advance