They look like some type of coprinus/“ink cap” mushrooms. Some types are edible but probably not worth it. Also FYI I think they are poisonous when consumed with alcohol or even within a day or so time frame.

They’re probably just helping to digest the mulch and fertilizers. Don’t think they would harm your ¿hydrangea? there…

The answer to "why" is "To spread spores."

It's a sign that your soil has a healthy biome.

They look like their help your flowers so keep them while their happy

I've grown some fungus to eat for while and learned a lot from it:

Mushrooms are the reproducing system of the fungus. They come out of the soil if the fungus has not enough space or if it is slightly dying; I would leave them there safely because if they are growing mushrooms and your flowers are ok, then they are likely to be giving proteins to your flowers and not really winning againt their roots.

Fungus that can be harmful to plants doesn't really show up in mushrooms, they likely would make leaf by leaf to die and get yellow or dry; or make the body of the tree to go black putrid and rotten. If you see any of this signs then you should squish some antifungus meds around your camps.