Staging Areas house combat ready units. Upgrading the staging areas will increases how many units you can keep. These buildings can be used as defensive buildings.
Why is upgrading the Staging Area important?
Upgrading the Staging Area will allow you to keep more troops. Having enough free storage area for troops is important. This will enable you to continue training troops which allows you to gain XP from the completed training and level up faster.
Upgrade a Staging Area to level 9 as early on the game as possible.
Why build fewer high Level Staging Areas, vs. many low Level Staging Areas
The Staging Area takes up an enormous amount of space on your Bases that can be better used for other purposes. It is better to build fewer large Staging Areas instead of many smaller ones since doing so will take up much less space (and each Level higher holds almost twice as many Troops). This becomes particularly important as you get your HQs upgraded to Level 12. See Base Design: Complement for more info.
Note: Do not waste your time and Staging Area capacity by training for more than 25 days. By doing so, you will level up by only 250 levels no matter how many troops you train.
One exception to a 25-Day limit (but not the 250 Level Limit) is if you intend to use many (more than a dozen) 30% Training Accelerators, then it is advantageous to build for as long as possible (up to a base training time of 400 days).
The first few Training Accelerators you use will decrease your training time by an enormous amount whereas the last few will provide a 30% reduction of a small unit of time (IE: not much), so it is best to refrain from using any Training Accelerators when you have less than one week of time remaining.
With so many Troops available, at a reduced cost, it is painless (lower cost) to drop them on the Map (or Tournament Map) to obtain XP, FP, and Upgrade the Map Node for better defense. You still only get 250 Levels (unless there is a '1000-Day Build Event', then there is 1000 Level Limit) but it reduces the number of Training Accelerators you need to produce the same number of Troops. You also get XPs and FPs for dropping the Troops (but not much).