What’s the purpose of the Reserves?

We (the Army Reserves) are an inexpensive way to maintain highly capable forces available, if needed. You'll find that many support functions like; engineers, truck drivers, doctors, etc are far more capable on the reserve side, due to the high compatibility with their full time civilian jobs.

The National Guard is very specific, as they actually work for their individual state Governors. Many legal differences (Posse Commitatus) in how they are able to respond to disasters and civil unrest. The National Guard has its roots in colonial militias, whereas the Army Reserves is primarily designed to support the Active Duty with additional capabilities.

To augment the total Army force. Reserve and Guard Soldiers make up the majority of the Army's total strength... by a lot.

Well think of it this way. You have the Active Duty component. Strong, fit men and women ready to go at a moment's notice. Now what would you do if the Active Duty component wasn't enough? Then you start pulling from the Reserves. The Reserves for a lack of better words is our back burner just in case bodies.

If we had the draft, we'd be pulling from a mixed bag. Some fit, some fat. Some healthy, some sickly. Then we'd have to spend our time training them just to get them up to speed.

Instead we have the Reserves components. Men and women who are already dedicated to fighting on behalf of their country. They are already trained and receive continuous training a couple times a year to keep them ready.