No "Saggy A..s" does not only refer to looks of your bottom, it refers to the Strength and function ability of your pelvic and hips. It helps the lower extremities function better.

All the muscles on the body were put there to perform a specific task in movement. If some muscles are weak, then other muscles on the body try to do work that is not theirs.
This leads to pain and weakness in the body. The more you can make all your muscles even the better the body functions.

We all want our glutes to be nice and fit.  I hear girls all the time comment how they hate their big butt or flabby butt. Getting your glutes into shape as well as the rest of your body will help you feel and look better, but also keep your bones stronger.

We have all seen older people having "saggy butt". It's not pretty. Think of the butt muscles, your Gluteus.   Gluteus is short for Gluteus. You have the Gluteus Medius, Gluteus
Maximus and the Gluteus Minimus.  See Pictures above.

All three of these muscles play a roll in everyday function ability. When our muscles are stronger our bones are stronger. 
If the glute muscles are loose, weak and saggy then the pelvic bone will be weaker.  We usually hear of older people breaking their hips.    They call it "old lady saggy butt" sorry to say.... but.. people say it...  If the muscle is saggy the bone is WEAK.

If these muscles are weak then you get more back pain, because your back is trying to do the job the glute muscles were created to do. Look at where the muscles attach on the bones in the pictures above. Back pain, hip pain and lower body pain, including knees and ankles can be from weak glutes.

It is also a larger muscle group and you burn more calories when you work the larger muscle groups. 
Take care of you.
Plan for your future. 
Your body strength is an amazing goal for a future.