A lot of people run their entire business directly from their WordPress website. Having a plugin that allows quotes, invoicing and payments in the same place is quite convenient for them.

I built this because people were asking me for it before I sold my previous software business. I know tools like WaveApps (and countless others!) are really useful but I think invoicing built into WordPress does meet a need.

I hear ya, appreciate the follow up!

How does wave make money if most of it's services are free? Do they sell your data?

I've used Wave before, they have a payments solution and I think they charge around 5% fees per payment taken on your behalf. At bulk you can usually get fees down to the 1% mark so they'll make their ~4% per invoice paid via their system.

I think they also have a paid support option but I can't remember, it's been a while since I used them.

They explain it pretty clearly on their site.