The significance of wage employment to labor unions is that wage employment is detrimental to unions and is, therefore, something that they tend to oppose strongly.
Wage employment involves employees who are not full-time employees of a firm. Wage employees are used in temporary positions. They are used when a firm needs employees for a short time (such as in seasonal employment) or for a limited number of hours. This typically means that such employees do not enjoy the benefits that full-time employees do.
The hiring of wage employees is bad for unions. Wage employees are not full employees of the firm and therefore are not generally union members. Firms will, at times, try to move more of their work to wage employees so that they do not have to pay the higher costs associated with providing the full wage and benefit packages that full-time employees command.
Therefore, wage employment is a significant tool that can be used against unions. This is why it is significant to labor unions.