They wanted to prevent them from joining because they couldn't bully them if they managed to join. I think that answers for both.

Now your turn to answer my original question, please.

Why do you think Russia invaded Ukraine? Like, what is their primary goal. The impetus that drove them to approve the invasion.

Because they want to become a global power once again. That is their dream. They want to be imperialists themselves, but unfortunately for them, they have failed to do so. In the past decades they have poked their noses into Africa and the Middle East, with some success. But simultaneously they have lost their grip on regions they previously considered to be under their imperialist umbrella. It started with Georgia, which they solved with violence. Next it was Ukraine, and then Syria. And then all the unrest in Belarus. They got spooked that their imperialist dream was failing, so they went in to change the regime in Ukraine. But that didn't work out as they planned. And to top that, they also lost their foothold in Syria completely. And now, just recently, they are losing Azerbaijan too.

Secondly, what do you think the functioning role of NATO is?

Honestly, it is to protect US geopolitical interests in Europe. Making Europe depend on the US for its defense. But it is not that bad of a deal for Europe, as it keeps the peace (in "western" Europe).