They're sorta like On/Off levers that have yet to be assigned a use by you. Suppose you want an enemy to appear in a certain place, but only AFTER the player has completed a certain objective. In that case, you'd use a switch (and I DO recommend renaming them. For this example we'll call it ENEMY SPAWN) to make sure that the enemy only appears after the other objective is finished. You decide that the trigger for the enemy spawn will only work if the switch is "Set". You make a trigger that has an action of "Set ENEMY SPAWN" once they've completed that first objective. Up until the objective was completed, the enemy wouldn't spawn at the location because the switch was set to "Clear". But now that its "Set", the enemy will spawn.
I hope that makes sense. It was a bit of a hurdle for me, too.
some editors use switches. some use death counters.
use switches when you want 1 event to happen from various conditional statements. switches can save you much triggering. for instance, if there are 50 ways of doing the same thing; lets suggest every time you bring a unit to locations 1-50, it displays a text message and creates a unit. instead of triggering those actions 50 times, you just set a specified switch each time. then, you have 1 trigger which executes all the actions and then turns the switch off again.
CONDITIONAL TRIGGERS
Condition
If any player brings at least 1 unit to location 1
Action
Set switch 1
Preserve trigger
Condition
If any player brings at least 1 unit to location 2
Action
Set switch 1
Preserve trigger
etc, do this however many times you like
RESPONSE TRIGGERS
Condition
If Switch 1 is set
Action
Clear switch 1
Display text message
Create unit
Preserve trigger
that was just an example of using switches. there are other uses of switches, like randomizing and structuring complex code, but the more you play with them, the more you will get use to them. they are there just to help your system decide what to happen next.