In situations where you have space CPU power but not spare GPU power because your GPU(s) & VRAM are allocated to be busy on other tasks, you might prefer to use what you have rather than needing to upgrade that will cost (even if that means the task will run more slowly).

If you are wanting to run this on a server to pipe the generated speech to a remote user (live, or generating it to send at some other appropriate moment) and your server resources don't have GPUs, then you either have to change your infrastructure, use CPU, or not bother.

Renting GPU access on cloud systems can be more expensive than CPU, especially if you only need GPU processing for specific occasional run tasks. Spinning up a VM to server a request then pulling it down is rarely as quick as cloud providers like to suggest in advertising, so you end up keeping things alive longer than absolutely needed meaning spot-pricing rates quoted are lower than you end up paying.