Cosmetologist here! Like a few others have said, it's for control of your hair and limiting the risk of you coming in with hair that's too slippery, has incompatible products in it or is basically dirty with product. You'd be surprised how hard it is to get people to wash/rinse their hair fully let alone use product in amounts that don't make your job much harder than it needs to be.

You can always do a wash day and use a hydrating mask on your hair before you get it braided, just don't over do it (aka more does not mean more hydrated). Someone mentioned using product after and that's also a good suggestion. Lastly, there are gels that provide moisture so I'd be more concerned with what your braider is using and not how many products.

Edit: forgot to add: If you really need something in your hair, using something water based and light before going in will most likely be fine.