Inbetween there's fundraising.
There's no interest in turning campaign and party organizations into actual membership organizations, and to give the members something to do in the inbetween times.
Most people don't really want to do anything, but some do, and many more would appreciate the illusion of doing something a bit more than 'retweet if you think this important.'
One could go a bit further and commit the ultimate sin of having membership organizations that actual benefit the members in some ways, but this is, according to the lanyards, the most corrupt form of politics.
But at least make people feel involved. Cynically, giving those people something to do means they aren't finding their activist outlets elsewhere and coming up with world-destroying slogans like "defund the police." If you want to influence people who aren't on the payroll you'd better spend some time influencing them.
I'll keep saying this, I guess!