Actually quite often. I'm very surprised users do not use unique email addresses when contacting different companies or vendors to protect against spam if one of your email addresses gets leaked after the company is hacked.
For example, I would use email address A for company A, B for company B and so forth. If company A ever gets hacked, I can just block any emails going to email A and not worry about playing whack-a-mole with the 100s of email addresses that spammers will use.
I can also use the email addresses as a way to auto filter or apply inbox rules.
I'm very surprised users only use 1 email address at a time especially if your email gets leaked once to spammers, you have to start dealing of spam or hoping your email solution has very very good anti-spam measures.
For me, this is just a normal usage of an email client.