It's easier and more secure for your users.

Users are very likely to already have an account at a social media service such as Facebook or Twitter, or have a login with an Operating System provider such as Microsoft, Apple and Google. Users are likely to be already logged into one or more of these accounts already. Using them to logging into your site, or even create an account on your site, removes a lot of friction.

Further to that, these accounts are also very likely to implement multi-factor authentication. This makes logging into them (and by extent, using the to log into your site) much more secure than the typical username and password authentication in Joomla.