When everything is one big screen full of white text, I find it very difficult to visually parse out, separate, and focus on information. Colors help guide your eyes and provide context. They can certainly be overused, however.
Why? I don't think emoji are necessary, but they are just Unicode characters; the only objection I can think of is that they are "too playful", but if our OSS CLI tools, written as a labor of love in our spare time, have to be "serious", we are utterly fucked. Unix hacking has never worn suits and ties.
Unless we are talking about unicode support. Indeed, the software should make a basic inquiry to see if the shell/terminal emulator supports unicode and fall back to ASCII if not. But there is a difference between "I don't support unicode" and "my unicode support is broken": the latter needs fixing, and emoji are actually a good test case to see if you really support unicode.
A little bit, as I find that it can make me lose my trail of thought. Red errors for LSP or because something is wrong in the executed command is great though.