Your Gmail can be used over and over again, with no one the wiser.
You can append to your existing e-mail address in various ways, and this could be pretty useful for seeing who leaked your e-mail address to spammers. For example, for your bank, give them the address myaddress+banking@gamil.com. Then, if spammers send to that address, you can quickly see where they got the e-mail address from!
I've tested it with Proton Mail, and it works in exactly the same way.
Unfortunately, many companies incorrectly validate e-mail addresses. Sometimes you aren't allowed to register and sometimes you are able to register but then some things don't work.
Yes I must say I've only had about two that would not register a plus address. Most others are just sending and then accepting a OTP response. Plus addresses are also working with my own domain e-mail.
I use aliases on my own domain. For example, for lemmy I might use lemmy@mydomain.com or for my bank i use mybanksname@mydomain.com. Everything goes to the same inbox. There have been a couple times with job applications where I've had to reply and then they find out I'm not really indeed@mydomain.com but I guess I could set that account up if I feel the need.
I used to work for a product comparison company (think finance and insurance). We used to save the email address as typed for login and also with everything after the plus removed separately. For Gmail and certain other large providers, we also stripped out any dots e.g. a.j.uniquename@gmail.com became ajuniquename@gmail.com.
Sorry, I thought the rest was implied. Because the company also sold user data (and stated that in the T&C's). The industry is very aware of email aliases and so it is more valuable to have sanitized data.