Why did the Empire bother to compact their garbage if they just ejected it
into space anyway?
-AMeanCow, Oct 2014
Discussion
Space tidiness.
"I need you to get aaaaall the way off my back about the garbage compactor" - Screenwriter Guy
The human mind has a hard time conceptualizing the ostensibly infinite.
In short, mind clutter.
Small airlock doors, obviously.
Ejecting garbage into space during battles and fleet maneuvers could damage their own ships, so they had to be prepared to store it for hours or days. But trash is bulky, so they compacted it. Also, maybe compaction was followed by incineration or some other processes to destroy any potentially classified material that accidentally got thrown away.