Usual caveats that I'm not a diplomat, don't have all the facts, etc etc., but this doesn't seem like a rational move for Finland or NATO. The UK just yesterday agreed to protect Finland in the event of an attack, and the UK's involvement would likely bring in many other NATO powers anyway, so arguably they're getting protection they'd otherwise have to pay for for free. And actually, officially joining NATO will hav…
1. The invader would suffer a massive penalty for their reputation: didn't stop them from invading Ukraine.
2. The invader would be hit with massive economic sanctions: see 1.
3. Even unassisted, we would be able to mount a defense that can cause major losses to the invading force: see 1.
4. Even though there's no formal defense pact, surely the west would come to our aid: Putin says "serious consequences, remember we got nukes", west backs down.
And as a bonus:
5. If, against all odds, they decide they still want to invade and we get no help from the west, there's no point to actually fight back because they'll crush us like a fly in the end, so we should just surrender immediately: looking at Ukraine, this is no longer obvious.
Usual caveats apply, but I believe the main benefit for NATO is to prevent point 5 from happening.