Okay in all fairness the Finnish assisted the Nazis in the siege of Leningrad AFTER the USSR invaded them. However, the invasion of Finland was done in order to create a better defensive position for the anticipated Nazi invasion. Motivated largely by the already existing fascist sympathies of the Finns and their proximity to Leningrad.

It's hard to say it was worth it imo. The invasion went poorly for the USSR, it didn't successfully secure Leningrad, and it did heavily inflame fascist sentiment in Finland all while making the USSR look like an unreasonable aggressor to Western audiences and weak to fascist audiences (possibly incentivizing the Nazis to invade earlier than expected). If the Finnish weren't going to ally with the Nazis they definitely were going to after that.