MailCatcher runs a super simple SMTP server

If you're building a system that sends out emails, you generally can't send emails locally. So you instead use a service like MailCatcher/MailHog/MailPit/Mailtrap as an SMTP server that will "catch" the emails as they are sent. Then you just open that app up and you can see what emails were sent, what their content is, who they were sent to, etc. Some of these services also include email evaluation tools for things like identifying unsupported HTML/CSS, checking image/message sizes, headers, etc.

The other use case is when you're in a staging environment. You should generally seed such environments with fake email addresses but you can never be sure those emails are truly fake, and you can never be sure what email addresses your testers are using. So you set up MailPit and A) you never send a real person a fake email accidentally and B) all testers can see all emails.