Thenderbird
 

What does Thunderbird actually do?
I happily have my email with gmail. Does Thunderbird download all my mail to this laptop? I don't really want that.
It just that, while making a webpage, if I use mailto then try it, up pops Thunderbird and wants to be configured. I don't want thousands of emails on my laptop

Thunderbird is a mail reader. Any mail reader will tend to get your mail and move it to your local machine.

There are various ways of setting things up to not download the body of a message until you want to read it.

If you don't want anything downloaded (except attachments) then use webmail.

Thunderbird can be set up to leave the messages on the server and just download headers or to download the messages and remove them from the server and everything in between.
Thunderbird is an email client. You may prefer webmail but having a proper email client does have its advantages including being able to retry both send and receive if your internet connection is broken and being able to view and interact with more than one mailbox at a time.

Downloading e-mails to your local machine is a nuisance. I use Thunderbird as IMAP client which leaves all mails on the server and just downloads the headers. It is much faster to use than webmail. And new messages pop up immediately. You can use Tbird as an IMAP client for gmail. Try it and see the difference between webmail and an e-mail client. You can use the same mail boxes you already have in gmail. You could even use Tbird on some machines and webmail on others where you have not installed gmail.

jlinkels

Right up to the point that your mail provider goes down, or somebody slaps a pointless DMCA take-down on your account (for example), and you want to reference a message.
Sorry, I agree that IMAP is generally a better solution, but it's worth considering what you use and why.