In the employer/employee relationship, the employer has much more power. This makes any negotiations, such as over wages or working conditions, favor the employer before negotiations even begin. By joining together, employees can combine their influence to level the conditions for negotiations.
An employer that does not like the way negotiations are going with employees can threaten to just close the company, or relocate it. This gives the employer incredible leverage in negotiations. The ability to strike gives employees a threat of almost equal power (strikes are generally temporary). The more power employers have over employees, the more lopsided