Why require new users to change password?

It would be convenient for me to have them keep their initial password because I am in a smaller business setting, and if I know their password I can set up Google Drive Sync and Google Apps Sync for them, instead of having to write instructions for them on how to do this themselves.

This is exactly why it is good practice to require them to change passwords after first login; they should be the only ones who know their passwords. It eliminates any potential for administrative abuse, or the perception of abuse even if none exists. Additionally, passwords they choose are less likely to be forgotten and require reset, or to be written down where they can be stolen and (again) abused.

Your second question about period password changes has already been answered in the following question: How does changing your password every 90 days increase security?

Forcing users to select their own password at initial logon, (the first time they authenticate), ensures that NOBODY else knows the password for the account once it has been changed.

This is a control process called single-control. Single-control means an resource or access to something is maintained by a single-individual. Since the userid/password combination is maintained under the user's single-control, it strengthens accountability in that we can identify with a fair degree of certainty who logged into what and when. If the password is compromised, (no longer under single-control because multiple people know the password), then we have less accountability because we cannot be certain who logged in with the userid/password.

Single-Control practices also protect those who do not have access to the resource or credentials in question. Since you should not have access to the user's password information, it keeps you safe in the event of a breach. If you do not force password changes, it is possible that you could be viewed as suspect, even if you have done nothing of the sort. EDIT: (This is called risk exposure, where not following the single-control process results in exposure to additional risk)

It is imperative that we enforce and maintain control-processes so that everyone stays safe and secure, and we don't expose them or ourselves to unnecessary risk.