Big VPN providers try to create this impression that your ISP can see everything you do, and that you're completely exposed without a VPN.
This is misleading. Nobody can see the actual content or full URL thanks to HTTPS/TLS,
only the domain names are exposed (even with Encrypted DNS). This is by design, otherwise data packets can't be delivered, like an envelope without an address.
For example, if you visited https://www.youtube.com/watch?v=ewf-8rx9_uQ,
no intermediary can see which page you visited specifically - they only know you visited youtube.com.
While a domain name may not seem like a lot, over time, it's data on everywhere you go and when, which is worse when your internet provider also knows your identity. This is typically only a concern if your ISP is also your government (as is the case in many restrictive countries), or if your ISP has been known to monetize this data.
If you don't want your ISP or network admin (e.g. a hotel) knowing this, a VPN revokes their ability to see it.
This isn't because VPN encryption is 'better' or 'military grade' (contrary to what big providers want you to believe); it's simply because the whole data packet (including the address) gets encrypted and put in another data packet, with the VPN server as the address. In this sense, a VPN server works just like a package forwarding service, relaying the inner packet for you.
If you trust your ISP (for both home WiFi and cellular), then this is not a concern for you.
Remember, one does not simply 'gain privacy' just by enabling a VPN — there is only a transfer of trust. Privacy is only gained if it moves to an entity you trust more. So instead of your ISP being the one who can potentially see your domain names, your VPN provider becomes that entity, because they are now the ones relaying the inner packets for you.
This is why trust is the most important factor when deciding which VPN provider to go with — and why you want to be cautious of any free ones, because running VPN servers costs money. Remember, if you are not paying for the product — then you are the product.