What is the purpose of linking the notes app to mail

That associates your notes with your email service provider - houses your notes there. So, for example, if you accidentally delete a note and would like it restored, then if that switch is toggled “on”, your email service provider’s policy would determine if and if so, how that restoration would occur.

Your question is extremely unclear.

I can make one guess what you’re asking. Perhaps you mean:

“I understand there is an option to house my notes in my email account. What purpose does this serve?”

Choosing where to house your notes is extremely important. For one thing, it determines which of your devices can access those notes. For another, it chooses the backup method for those notes.