Honestly ... if the built in backup does what YOU need it to, there's no reason for YOU to look for a backup solution since you've already found what works for you.
On the other hand, for many people the built in solution either does not work as they would like it to, or they have slightly different needs that prompt the desire to have a backup solution.
There's some good reading to be found but
THIS is a decent run through of why ... and
HERE are some excellent reviews (and reasons for backing up as well)