Because it protects you from ISP or targeted MITM. Lets say your ISP decides to spy on users or someone cuts into the internet wire going from your appartment to ISP, without VPN they can fully see and modify all http traffic as it's totally unencrypted, in https traffic they can't see the content but they see domain names of sites you are visiting and exact time when you are visiting them. With VPN 100% of your traffic is encrypted and in similar situation absolutely nothing is visible or modifiable. Someone can MITM on VPN provider itself, but it's not really suitable for targeted scenarios, because those wires are in other part of the world than you, those wires are much better physically protected and even if malicious actors succeed they will have very hard time filtering traffic from different users from each other.