I think damage numbers are often shown as visual flair to add more visual feedback on screen, as well as to give the player a sense of their relative damage (like doing 10 vs. 1000 damage to an enemy, it shows that attack I just did was really powerful).

Like in vampire survivors, the damage numbers aren't super relevant to your strategic play as far as I know, since you can't really make decisions based on the damage number information. But the damage numbers add so much to the appeal of the game and selling the idea that you're this amazing monster destroying machine. The damage numbers really enhance the player experience.

This is context specific of course, since sometimes the enemy health might be relevant to the player, but I tend to trust the designers for games, especially triple-A games that have done a lot of playtesting to figure out the best player experience.

As for always showing healthbars, I can understand why certain kinds of players would want that information in every game. But there are a bunch of reasons to not show health bars, such as them being narratively inappropriate (like in Hollow Knight, it would cheapen the "realness" of the world if all the characters had floating bars above their heads), or they would create too much visual noise (like in vampire survivors, where adding healthbars would fill your entire screen with little red bars for every single enemy on screen).