VPNs have several purposes but the big two are hiding your traffic from attackers on the local area network and concealing your location from sites that you visit.
If you’re using a VPN on wifi at a cafe and anyone else at the cafe can run a rogue DHCP server (eg, with an app on their phone) and route all of your traffic through them instead of through the VPN, I think most VPN users would say the purpose of the VPN has been defeated.