A VPN is basically just a replacement ISP. They see everything your ISP traditionally sees.
Many ISPs are already selling your traffic data. So if you trust that your VPN isn't selling that data, then you'll be more private using them. And if the VPN is in a different country, then your government is less likely to be able to force the VPN to give up data.
You can defend against timing attacks and other traffic correlation attacks too, using techniques like cover traffic.