Gemstones is that one currency that you actually have to work for. A few games have them, to reward players who actually make the effort to obtain them, whether or not they spend actual money. Money can't buy everything, don'tcha know.
At the end of the day, this is a strategy game, and lots of thinking is involved. So, why not make it worth their time? I'll take inspiration from what the other spiced-up players have said countless times, but the purpose of Diamonds is to think a little less.
You messed up a negotiation because you didn't use your resources wisely? Diamonds. You moved your Units in the wrong places and aren't strong enough to face the next wave? Diamonds. Ran out of space to place Buildings in? Diamonds. Spent Event Currency non-efficiently and now aren't gonna be getting the Grand Prize for free? You guessed it: Diamonds.
Basically, almost all of the times that Diamonds are used, it's to get something done right away or to correct something that resulted from a player's poor judgment. An advantage that can only be bought with money, which is a fair deal. So to avoid giving too much advantage to paid players, things like Gemstones exist. Something that must be worked for, something in which both F2P and premium players have to put in effort; again, this is a strategy game.
Stop being lazy.