That space is required to run the games. They contain the vulkan equivalent shaders that have been translated from directx, this has to be done for each material shader in each game. Unless they are stored somewhere, it has to be done again every time you run a game, causing stutter.
Shaders will need to be compiled either way, not downloading pre-cached shaders just means the deck will create them when you run the game, using the deck GPU to do it instead of steam servers. It will then store them for later use, using up the storage anyway.
By turning them off, the only difference will be that the deck is now wasting battery power on compiling the shaders on its own, rather than grabbing them pre-done from steam servers.
You might get away with not downloading them all, if you only play one or two games, but this will mean that not all installed games are actually playable, as you’ll run out of space for shaders if you used up the space you “saved”.
It’s worth noting that this is only a thing on the deck. Valve does not provide pre-compiled shaders for any other GPU. (Though you can tell steam to compile them in the background so your games are always ready to go.)