It depends on what other market forces there are. If the utility has a government mandate to install more renewable energy than there are renewable energy customers, then it is like you say - just a book-keeping exercise.

On the other hand, if there is no government mandate, or if customer demand for renewable energy exceeds that mandate, then it provides a profit incentive for the utility to install/purchase more renewable energy.

One of the British '100% renewable' companies used to buy renewable power and then destroy their surplus resellable renewable obligation certificates instead of selling them (each supplier was obliged to hold certificates covering x% of their supply, where x << 100). That had the consequence that the renewable power generated was genuinely additional, as the other generators would have to get their renewable obligation certificates from new generation rather than the secondary market, at the margin.

They stopped, publicly reasoning that the proceeds from selling certs could be used to build their own additional renewable generation. I suspect it was just leaving too much money on the table, though.

Well, it makes a difference what kind of energy supplier turns a profit from your energy consumption. If you're buying renewable energy from a company that still invests into fossil fuels, it's fairly useless. If you buy it from a company that mainly builds up renewable energy production capacity, that helps the transition.

Of course, even a 100% renewable supplier needs to buy from the market to smooth out peaks in energy consumption for their customer base.

(disclaimer: i work for such a renewable energy supplier)

Some providers promise to build new renewable capacity that matches the consumption of their customers (not sure about the timeframe). I'm not aware of any provider that builds the required storage too, though