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.

I am a casual American football viewer but my understanding is that the kneel ends the current play but keeps the clock running. Each team has something like 40s to setup their formation and snap the ball after the previous play has ended. If the game clock is still running (this is concurrent to their 40s of "setup time"), the team that is in possession of the ball can just use the full setup time (idk the formal term for this) to just run out the game clock.

Each team has 4 attempts to move the ball forward 10 yards, where if the ball moves >= 10 yards they get a fresh set of 4 attempts. These are called "downs".

If the team has any downs left when they kneel then they can maintain possession of the ball and can thus run out the clock. Most (all?) of the time the teams end the game even if there is time left on the clock.

Note that either team can call a timeout pre-snap which freezes the game clock. Certain plays also result in the game clock freezing between plays. There is also a 2-minute warning at the end of the 2nd/4th quarter that also freezes the game clock.

IMO clock management adds a very interesting strategic layer to NFL football.

In baseball it is possible to hit a home run without the ball leaving the playing field. It's happened several times in the past 2 months.