A lot of times when people ask about communication issues, it's like there's a horde of people who come to say "just say what you mean". IT'S NOT THAT EASY.
Yes if your flatmate puts smelly food in the fridge you CAN just ask them to move it instead of ranting on reddit, but if one of your friends wants to wear a shit outfit to your birthday dinner, it's not that simple. Subtlety and tact matter.
PEOPLE CAN MISUNDERSTAND THINGS. "Just tell them" doesn't work when they might think that you meant what you said in the wrong way, or that you don't think well of them, or it was an attack on their taste or so many other things. Being understood clearly is a valid thing to be worried about, and sometimes the best way for that is to be a little less on the nose. In my example, there would be so many people on here who would encourage me to say "I don't like what you're wearing, please change it" unironically.
Straightforward, cold therapyspeak is not the answer for everything.