All of those things -- POP3, IMAP, SMTP -- are used for non-webmail email. If you only use Gmail as a web email service, you don't need them.

POP3 is for downloading email from the Gmail servers to your PC using a "traditional" (i.e. not web) email program. SMTP is for sending email via the Gmail servers using a traditional email program. IMAP is for reading email using a traditional email program, but, unlike when using POP3, the email data itself remains on the Gmail servers except for things like attachments that you download explicitly.

If you used email programs such as Eudora, or Thunderbird, or Evolution, or Outlook Express, you'd be using SMTP and either POP3 or IMAP.

Gmail offers access via those methods not because it's an essential part of being a webmail service, but because some people might like to use the Gmail service (when away from home, say, or to take advantage of its antispam features) while keeping their existing email program.

Peter