All starters with on-board solenoids that I know of use them to engage the pinion gear to the ring gear, so without the solenoid, the starter cannot crank the engine.
The other function of most starter-mounted solenoids is to be an electrical contactor, which most airplanes do not take advantage of as they still employ firewall-mounted start solenoid/contactors.
Every one of the several Skytec solenoids I've had fail were from the plunger becoming sticky, or galling, so that it no longer slid into its housing easily. My opinion is the stickiness is due to dust and debris being slung into the starter nose and then migrating onto the solenoid shaft. Cars don't have that issue as they have a dust shield to keep the flywheel area clean.