Leia showing up at all was pure fan service, and really makes no sense if you think about it. What was a senator doing at the Attack in Scariff? … She wasn’t fighting nor did she have any expertise
What makes you think she’s just a senator? That may be some EU stuff, but her actions in the OT show that she was involved with the Rebel military:
-in SW she’s on an undercover mission for the DS plans & is some sort of strategist before & during space the battle.
-in ESB she briefs pilots of their mission & helps carry out the entire escape from both Hoth & Bespin.
-in ROTJ she goes on 2 different covert military missions & in both she plays a major role.
She’s not just a politician/princess.