It's to prevent a bait-and-switch on behalf of the employee and situations where a person is "employed" by a company only so they can immigrate and then do some

It's to prevent a bait-and-switch on behalf of the employee and situations where a person is "employed" by a company only so they can immigrate and then do something else, on behalf of the employer. We can easily let people switch job titles and employers while addressing potential loopholes though.