nah, it's not because catnip is mimicking sexual pheromones, at least that's not what people have said or what ive heard on the research ive done. Catnip may have different effects on every cat, only 50% of cats don't even react to catnip. It also has different effects on different cats. Your cat may have go crazy, while your neighbors cat may simple have slightly a little more energy. Some examples of why people may give their cat catnip.
1) Excersize. Some cats may be more overweight then others, therefore some owners may give their cat catnip to get them to increase energy levels for more excersize.
2) To enforce playfullnes. Some cats may seem depressed, or really grumpy, some people give them catnip to try and get them into a more playful and happy state.
3) For lauphs. Completely immoral and totally wrong(in my opinion) but usually the cat will show when it has had enouph catnip and usually will refuse any more offers.
In conclusion, catnip can have it's possitives, but if you think that catnip is having negative effects on your feline friend leave the catnip aside. Since they have different effects on different cats, it can be good or it can be bad, but cats will not overdose on catnip and it will not harm them eather, it's a herb.
In my opinion catnip is unneccesary if your cat is healthy and energetic already.