Prevent installation of malware, spyware... Admin password requirement
If other people use only the guest account, then they have no privileges to install anything.

I have added another account (beside the admin and guest account) which I use for daily work. The reason: sometimes someone ask me if he can use my computer for a short moment, and this way I do not have to switch to the guest account.

And because this third account has no admin privileges, it is more secure than using an admin account for daily work.