The main reason to perform Skate or Long Skate is simply to buy time. If you have a mission tasking like TARCAP (providing cover at a target which other aircraft are striking), then your primary goal is simply to protect that target, rather than score kills. In a situation like that, you usually have a designated time period where the strike package will be vulnerable, and as long as you keep all enemies away from the target during that vulnerability period, you have accomplished your mission. In that case it makes sense to launch your missiles as far away as you can, just to slow the enemy down. At those distances, as you pretty much know that the enemy will not be hit by your missile unless they willingly choose to fly into it rather than defending, you can for example choose to only shoot a single AMRAAM at an enemy 4-ship, simply to force them to defend. Then as you get closer and the probability-of-kill increases, you can increase the amount of missiles you launch as they now actually stand a chance of killing something.
The downside of the Short Skate is that you don't have a lot of options left after that. You're cold just outside the Stern WEZ, you probably have an enemy behind you, and if your opponent is faster than you it won't take long until they can kill you even from the rear aspect. If you have superior numbers you might be able to recommit doing some kind of split with your wingmen, or maybe you can flee for long enough that some other friendly flight delouses you or your opponent figures it's not worth the fuel to chase you down. However, if you've got a long station time to keep, then fleeing for a prolonged amount of time isn't great for your fuel state.
And yes, you can view all these distances as decision points. Nothing is etched in stone, situations change, and you've gotta be able to react to those changing circumstances. Like in the example above with a TARCAP mission, if your only goal is to buy time and you've planned a Skate, but you get a golden opportunity to actually kill the enemy, then that is obviously preferable to turning out and spending an additional 10 minutes wasting fuel and missiles just to buy time, only to eventually end up in a position where you need to kill the enemy anyways, just with worse odds, less fuel and less missiles than you had before. The important thing is just to make sure you accomplish your mission task in the best way possible, that's what your decision making should be based on.
WHOGX5's post in How do I properly execute a skate? was marked as the answer