cause most of them don't really want pay your expected salary or market price

If you ask for 4.5k but your previous salary is 3.5k, from the payslip they will just offer you 4k.

Of course it is up to you whether to accept it or not but most company want payslip so they just offer you slightly higher than previous salary

Knowing your salary will allow them some negotiation power, same reason why most company says you are not to share your salary with colleagues

If you know the market rate, you have negotiation power.