The responses so far seem either irrelevant or insulting. Again, asking for iOS Roundcube-app or mobile-friendly Roundcube-login-website to access mail stored on one's host server (because all hosts I've seen lack mobile-friendly email interface for Roundcube)
Again, setting POP/IMAP/SMTP servers on cell phones is storage + security risk for many users. Obviously webmail useful for phones, e.g. this why Gmail-app exists for iOS!