What information assymmetry. Surely you can find out from your network what a typical salary for the position is. With sites like glassdoor you also can get a good indication of what you typically can expect. And by being savvy in the interview, you can find out of what value the position might be to the company and you can negotiate on those grounds.
Sometimes the employer is in a better position, sometimes the employee. Sometimes the employer needs to fill a position urgently, has got specialised requirements or the employee is gainfully employed somewhere else and in a much better bargaining position.
If you have a network. If people in your network talk to you. If people post to glassdoor about the company.
Don't make the mistake of assuming that this is true for everyone. Also remember that even if you have a network, if it is influenced by your gender (women with women friends) , or race( black people with black friends) you might not have an accurate picture.
Unless many, many of the members of your professional network are quite homogenous to your skill set, location and market sector, and unless a large number of those actually give you accurate information, you really can't rely on them to find out what a typical salary is.
And the value of a position at two companies depends more on internal factors than on a job title. Senior Developer at X simply won't be worth the same as Senior Developer at Q.
If you know internal numbers, you can maneuver the upper-hand as an employee. But the best you're generally going to do is a ballpark, and have to leave it up to the company to determine how much they want you vs. the other candidates they have.
Sometimes the employer is in a better position, sometimes the employee. Sometimes the employer needs to fill a position urgently, has got specialised requirements or the employee is gainfully employed somewhere else and in a much better bargaining position.
We really should stop playing the victim.