Firing can be used as a reaction so.....
Side A moves into contact with Side B
During "B" Reaction phase, B fires, maybe cause some Disruption to "A"
Then during "B" Combat phase, you resolve the Combat, maybe causing more DISR to "A", or maybe enough DISR to eliminate "A"
So assuming "equal" units, why not fire as a Reaction. Or am I a dumb bunny and that's what everyone does anyway?