They prioritize low-coverage areas (no idea how generous they are for covered areas).
Nearly all of their position data is from these receivers, so you're providing them with more coverage/redundancy for the plane positions.
The hardware is relatively cheap (under 50$) and I don't imagine the business tier costs them much money.
> I reckon if I shelled out for a business plan I could start doing some simple stuff with free tier / extra paid features
You don't need to they'll give it to you for free.
sidenote: You can build your own with an rtl-sdr and a raspberry pi/cheap computer. If you do this you can forward the data to all of the commercial sites for the free perks, and to all of the community sites.
https://www.flightradar24.com/apply-for-receiver