It's sort of odd thinking about the pace of MMO PvP or MOBA combat. In MMOs and MOBAs, combat is by default a damage race. If you're the player/team with lower damage, you want to run. With higher damage, you want to engage. Sometimes there's a bit of fighting game-style posturing to bait cooldowns, which IMHO is the best part of MOBA fights.
One of the big skills in MOBAs and MMO PvP engagements is knowing when you can win. If you have an idea what the enemy composition is, you can guess whether the enemy can win or you can. In MOBAs it's fairly simple since you have a good idea which teamcomp beats which, and you can check enemy items with the tab menu, and then barring stuff like 4v5s you know roughly who is going to win. Of course, the team that will lose wants to create a favorable engagement so that they can win, and the winning team can be a bit more aggressive since they will win slightly less favorable engages.
This leads to snowballing, which is kind of bad. Already MOBAs are super snowbally, so having a team slightly behind also leads to the enemy team wanting to fight more to increase their lead. It's not so good. I really like fighting games for this reason, because the person in the lead wants to preserve his lead, so unless an opportunity happens he isn't going to try and close the game out.
So in these types of games, engage/disengage is really important. You want to create localized imbalances so that you can either get more ahead/force a fight, or try to take the lead if you're behind.
Consider a 1v1 situation with a character like Blitzcrank. He will not generally win a 1v1 against most DPS-type characters, unless he's far ahead on items. His kit isn't really built for that. However, he has a lot of CC, and his unique grab lets him pull enemies into bad situations. So if he doesn't win in a 1v1, he might still be able to use his CC and burst to chunk off an opponent's life enough and then disengage. If the opponent persists in fighting and he can get his cooldowns back, he might just win once they return, assuming he can avoid taking too much additional damage.
For a more extreme example, we can look at Kha'Zix who has some DPS but is mostly a burst character. He also has a ranged harassment attack. He might not win 1v1, but his ranged harassment skill can be used until he can.
You can replace this with MMO characters and classes, but MMO characters tend to be a bit more complicated with over twice the buttons of a MOBA character. It's harder to say who wins in a fight if any CC is involved, even if we know the pure DPS of the combatants. In most cases we don't know the pure DPS of the combatants anyway. MMO PvP is a bit like MOBA PvP in hardmode, because there's simply more factors to consider, and you can't see the entire battlefield with your camera. Of course, in MOBAs there's more strategic elements. I'm not at all trying to say MMO PvP is better (it's not).
When you're going for an engage of some kind you're trying to neutralize some element of the enemy team. Of course, you have to plan for what the enemy team can do in response, and also weigh in on what exactly you're trying to accomplish. One big example is characters like Teemo or Nidalee who don't fight well, but can impact a fight in other ways. You don't want to fight with both teams at full HP with one of these characters, but if your team hits with two Nidalee spears or Teemo mushrooms you've basically paid for yourself and can go in. On the other hand, if the enemy team has those you basically want to fight right away and neutralize the advantage these characters have.
Obviously I'm talking a lot about LoL here but you can apply this thought process to pretty much everything that isn't fighting games. You either want to fight and damage race the enemy, or you want to create localized imbalances so that your opponent can't damage race you until they will lose. This localized imbalance might be a favorable engagement or zoning or some other advantage. It might just be that you have to abandon that engagement and go somewhere else to do something. If you're in GW2 playing WvW, if the enemy is all zerging somewhere you'd be dumb to try and defend if you don't have a bigger zerg. It's better to contribute in other ways.
I feel a bit scatterbrained today, so hopefully this was somewhat readable. I promise tomorrow (first new TSG) will be less random.