If terrible websites hadn't destroyed the utility of popup windows, and caused browsers to block them by default, the MSAL popup flow would be clearly better than using the redirect flow. It's a much nicer experience, and far easier to code for.

Ironically, I only learned it existed because I did some work with a Teams application that had a tab, and login.microsoft.com doesn't work inside of an iframe, so there you cannot use the redirect flow.