Anonymity. Now the VPN operators and people who are watching the VPN logs don't know your IP.

yes. and also if tor is compromised you have the vpn as a layer of defence.

and how do you get access to the VPN?

all VPNs need authentication and payments, otherwise your just connecting to another open network with the same issues as TOR.

if you make payments, and then auth on connection, they "know" who you are and TOR at the start is not giving you anything

1. Not all VPNs are paid, a lot of free VPNs and open proxies

2. Not all payment methods are connected to your identity. Mullvad for example accepts cash in an envelope, or cryptocurrency payments.