I always thought that 16 GB RAM are enough for any videogame. Suddenly I have noticed that there are new computers with even more RAM. Is more excessive RAM useless?

For the time being 16GBs of RAM is probably more than enough. With few exceptions it takes an incredibly poorly optimized game to require more than 8GBs. I go with 16 just because every once and a while a game like Doom will come along that requires a minimum of 8gbs of RAM but in my opinion play better with more than the bare minimum, so I don't think 16gbs is excessive or useless but definitely on the high end for gaming. That being said it's only a matter of time before 8GBs of RAM becomes relatively standard. At which point you'll definitely want to have at least 16.

16 is still plenty. More can be useful in very specific situations (maybe 3d modeling or such), but for 99% of games and applications you don't need 32...hell, most of them can function fine on 8.

Sure bro.

For gaming, Yes. And it should remain this way for a while yet.
For Multimedia creation / editing, you could go higher.

Anything higher than 16 is unnecessary for gaming at the moment, those computers with more would be suited to stuff like video editing and animation. 8 is still plenty for most games.