Why shouldn't I leave trees floating in Minecraft?
Sometimes, when I enter a Bedrock Minecraft game and it is loading, a tip shows up:
DON’T LEAVE TREES FLOATING!
I don't think this does any harm, so why shouldn't I leave a tree floating in Minecraft?
As far as I know and can tell, it doesn't do any harm; it just might make things a little more difficult and unseemly.
While the tip may have been added more as a joke than a survival strategy, there are several reasons why you wouldn't want to leave floating trees:
Floating (leaf) blocks can give mobs a place to hide from the sun.
The leaf blocks also contain resources.
Floating trees can cause monsters to spawn in Survival [mode]. https://www.minecraftforum.net/forums/minecraft/discussion/3069988-dont-leave-trees-floating?comment=2
Without the leaf blocks (barring trades) you won't get the saplings to replace the tree that has been taken down.
In multiplayer it's considered a bad habit. I can imagine that is because it ruins immersion. And according to some (many?) players it simply looks https://www.quora.com/Why-shouldnt-I-leave-trees-floating-in-Minecraft .
For whatever reasons they might prefer it as a token of respect to the people running a server.