@simrin-- Of course, hair mascara isn't going to give you the same results as dyeing your hair with permanent color. I have a couple of tubes at home but I don't use them regularly. It's kind of a quick fix for times you can't have your hair done like @alisha said.
It's also a nice way to bring about a change in your look for a short while. I really like the bright colors for this like purple, pink and blue. I have dark brown hair and a streak or two with one of these colors makes things so much fun. It's also great for special events and holidays like Halloween.
If you're worried about the stiffness, you can try it with an updo. It kind of works like hair spray that way and will keep your look together in addition to adding color.