When an actor or actress steps into the skin of a character for a while, they can become attached to that character. Barring reshoots & ADR, once they finish with their scenes, their character essentially dies. The actor may inadvertently dress like the character again if they supplied their own wardrobe, but the playing of the role has a finite lifespan.
With shooting schedules, actors come and go for the most efficient use of time on set. They weren’t there for pre-production, won’t be there for post-production, and are only there for a percentage of the time during production, so unlike a crew that’s there from beginning to end, they’re probably going to miss out on the last hurrah celebration of wrapping on the project unless they’re in the final scene.
Since I also count myself among the ranks of actors, I’ve been on both sides of this. Production can get crazy on set and you’re generally thinking of what you have to do next without giving much thought to milestones, so when I’ve forgotten to realize that the actor I just worked with doesn’t have another shoot date later in the schedule, the life of their character ends without much fanfare at all.
It’s a little thing, but it does mean the world to an actor to have it brought to attention that they are finished and leaving the film family. Your actors will feel appreciated (and we are a moody bunch, aren’t we?), you’ll have a little crew morale boost (even if it’s five seconds of everyone clapping, it’s a nice marker for the project’s progress). Plus most actors will hang around until they have been announced out as they wouldn’t want to leave accidentally assuming they were finished. So, beyond just a thank you and morale boost, it’s an actor’s cue to unclip their mic pack, go back to wardrobe, and get ready to step back into their own skin.