What do we think of 2 phases reviving boss ?

Sorry for the mediocre english speaking.

My question is : is there any value for developers and / or players to make/face a boss that, once his life bar depleted, revives and display a second phase ?

The beheaded monkey in Sekiro might be an exception. I'm talking about Malenia for exemple, or the Snake boss in the chapter 1 of Black Myth : Wukong. Once defeated, they revive and they have a brand new life bar. Wouldn't it be better if the second phase was at 50% HP ? Elden Ring DLC, for example, have no reviving bosses if i remember correctcly thank god

To me, as a player, there is nothing to gain from. It's just frustratring. That's all. And I was wondering if there was anyone enjoying them or having an explanation for that.

https://preview.redd.it/clnm8wg6zwge1.png?width=1920&format=png&auto=webp&s=3d362fe624a5ee7372d7e70181c5e3c98bd60e61