Essentially the player with the ball is going down on their own, ending the down, same as if they had ran with the ball and been tackled.

They’re considered down when their knee touches the ground while in possession of the ball (“possession” having a specific meaning, with regard to the rules). Again, this is the same as if they had been tackled. The only difference is no one forced them to the ground.

Taking a knee is not something that would normally be considered a good thing since you lose yards and a down.

As for why it’s not a delay of game, that’s likely because it does not delay the game any more than any normal play would. It probably runs down less time on the clock than if they played normally, but of course playing normally is riskier which why they take a knee. The idea is to simply run down the clock as much as possible without risking a turnover and then leaving the other team with too little time to score.

If the rules could be changed to disincentivize taking a knee I think that would be more interesting, but I’m not sure how you do that. It’s also safer in an already dangerous sport.

In other games simply taking an action that intentionally runs down the clock is delaying the game. It’s a ref’s call, and could be done here too.