I won't get into other core benefits of paying for mail (no ads, business model where you're the customer, etc). But...
You can
- have a personal domain and email address for as low as $15/yr (weird tld, small mailbox)
- have something more reasonable (domain+email for $45-65 a year with 20-30GB + various PIM functionality depending on provider and price)
- go all the way up to fully hosted personal cloud solution via something like Microsoft365 ($150/yr)
- self host everything yourself - $7-15/yr for the domain
If you use email heavily, and/or if you value having the option to self-host more things, then it's absolutely worth it.
Personally, I self-host my cloud drive with a freenas box in a very close to default configuration, and use Fastmail for mail+PIM.
If my provider starts sucking, I can switch easily. The more paranoid could use the more expensive protonmail or fully self-host! But what's important is that should I ever choose to switch, it will be a one-and-done thing that probably takes me about an hour of work!