In My book there is only 2⃣ reasons to stanna
To win
To prevent someone else to win

Well, there's also to survive I'd say.

To keep the game interesting.

It's part of the game. You backstab because you think it will make you more likely to win.

The reason you backstab is because it's a zero-sum game. After the neutrals are claimed, someone has to lose in order for you to gain. If your ally leaves himself wide open to attack when you're deciding which way to go next, he probably deserves to be the one that loses.

yes, but why do some people go "next" if they're not even done with their current issues?

To paraphrase the great Russel Ziskey from the classic movie Stripes "Never stab anyone in anger, unless you're absolutely sure you can get away with it.

Because on maps like World Diplomacy it means that if you were China and backstabbed India, Russia wouldn't trust you if you proposed an alliance - so it's not always good.

"yes, but why do some people go "next" if they're not even done with their current issues?"

I'd answer this in two ways, one for the beginning player and another for the intermediate player.

For the beginning player: You will improve your play very quickly if you always ask "what happens next?" I'm fortunate enough to be winning this war: what happens after I win it? As you get larger,this is more and more important, because while a 4-center Austria can quickly shift units from, say, attacking Greece to attacking Venice, a 10-center Turkey takes several years if it wants to switch a fleet from attacking Spain to attacking Rumania. So, the proper time to choose a new target is, given the chose, slightly too early rather than slightly too late. Similarly, if I'm a 7-center power and only need 4 units to clean up my current war, it's time to find something for those other three units to do, and if three units are enough to actually get two centers...

For the intermediate player, there's a second consideration. As a successful mid-game power, you should always be thinking about whether you can solo. The biggest disadvantage you have in trying to solo is that the whole board will ally against you. But, the biggest advantage you can get is a large alliance, which must coordinate effectively and with internal dynamics. For example, you might find that in order to hold a line, a Turkish unit needs to move through Russian territory and make him nervous. Further, small powers are susceptible to be asked what happens if they successfully stop you -- will their allies turn on them and reduce the draw? A 14-center power facing two organized 10-center powers is almost always a 3-way draw. A 14-center power facing five 4-center powers is much more often a win. So, if you think you're likely to be pushing for a solo, err on the side of allowing tiny powers to live in order to counterbalance and reduce larger ones. You often only need one adversary to make a mistake or get tempted, and the more adversaries you have, the more potential weak links.

I back-stab you because you have an ugly back. Who am I to say you have an ugly back? That's because you have been a jerk, a demanding, abusive ally.

Backstab doesn’t need a purpose; the backstab is its own purpose.
You don’t ask why a plague spreads or a field burns.
Don’t ask why I backstab.

My most recent stab was to prevent a solo. It's one thing to help a guy get big so that you enemies will suddenlt perceive him as a threat. It's another to continue to help him after they see the threat and beg forgiveness. You are an idiot not to stab then and stop the solo.

In one game I was England and stabbed France just for fun. It took 6 years for me to regret my decision.