But usually, when employees want this, it's for post-employment restrictions on the employee. Also at acquisition time: buyers will enter into employment agreements with significant or key employees (most commonly these folks are also the primary shareholders, so they're often in on the deal all along).
You might have severance pay provisions here as well; release of all claims by employee, etc. Employers, obviously, want to have this as small as possible, because employees will negotiate up from here.
Some folks (e.g., executives) will negotiate a provision where if certain changes are made (e.g., relocating HQ to Buffalo, major change of reporting relationship or job duties), they get to continue collecting their salary as though they were fired without cause.
Covenants not to solicit the employer's customers (including customer lists are property of the employer, etc.).
Covenants not to hire away existing employees.