I believe they are meant to supplement things heh. I think most supplements are money grabs that promise the world but deliver very little benefit.
I use fish oil ( because it’s lacking and the body just cannot make it on its own) and has very real benefits.
I use protein powder simply because I find it difficult to reach protein requirements from food alone.
I use vitamin d3 because yearly blood tests suggests I am missing this important nutrient.
And I take a multivitamin to round out micro nutrients that may be missing ( for peace of mind)
These are the only supplements. I personally use and reasons behind them.
I don’t believe any supplement will help you lose weight or gain muscle. Only one thing can do that.
Calorie deficit(losing weight$ calorie surplus( gaining muscle)
I will consume a post workout protein shake as well as a multi vitamin. I used to use Creatine and add it to my shake years ago, have not done that recently but in the past those were the only supplements I've used. Saw very good results, combined with a "somewhat" healthy diet and hardwork in the gym.