Groot uses money for the same reason anyone does. To buy things he needs or wants. Granted, when the comics or series interacts with the characters, we don't see him using very much money but that doesn't mean he doesn't need anything.
As we have reported in other posts, Groot comes from an advanced star-faring civilization and despite his apparently limited vocabulary, we know he is in fact, highly intelligent and may choose to use his funds gathered from bounty hunting to purchase resources, equipment, or other things we may simply not get to see.
We know less about the MCU Groot, but we can assume the pertinent parts of the character's bio are the same. He is intelligent, kind, friendly, and curious.
If Groot could speak intelligently he would tell you Groot is no mere wood! The citizens of Planet X, where Groot came from were an advanced starfaring species complete with scientists and a curiosity about life in the Universe. In Guardians of the Galaxy we are never told what happened to the citizens of Planet X, only that Groot is the last survivor of that species.
Groot's physical capacity for transformation, instantaneous growth and incredible durability ensure his survival under adverse conditions. He demonstrates an ability to spontaneously grow flowers, transform into vine-like barriers, and extend his limbs at will.
From: How does Groot's regeneration work
In an interaction with Jean Grey, we see Groot's internal dialog for the first time, showing intense mental activity and significant cognitive ability behind every "I am Groot."
For all we know, Groot uses his share to establish parks, forests, nature preserves or other recreational facilities on planets. Groot's internal life is simply one we never get to know much about and have only Rocket's interpretation of events.