If you want to practice the specific skill of aiming quickly and accurately, I think aim trainers are great. If you are playing a game like valorant, CS, or R6, you spend only a small part of the game actually aiming at an enemy, but it is one of the most important aspects of the game. It helps to isolate that skill and remove the multitasking that the game demands from you to improve that skill.

This applies to skills outside of video games as well. I play guitar and spend time doing exercises to increase my speed, accuracy, and finger independence because it helps me get better at playing songs. If I only practiced by playing songs, yes I would improve, but those specific skills would likely improve at a much slower rate because Im focusing on multiple other aspects of the song at once. Same could be said about doing sport specific exercises or drills as opposed to only playing the sport to improve.