The reasons that employees join unions are generally closely connected to their material desires. They do not generally join for ideological reasons. Instead, they join because they want the material benefits that, they hope, will come from unionization.
Employees tend to care most about things like wages, working conditions, and job security. These are the things that are most likely to be attractive about unions. Unions tend to bargain hard to keep wages high. They try to set work rules that are, essentially, meant to ensure that workers’ jobs are not too onerous and, importantly, that more jobs will be needed. In other words, they try to ensure that each worker will not have to work too hard, thus making sure that more workers will be needed. Finally, the unions try to make it difficult for employers to lay people off.
Because unions do these things, workers often want to join them. They do not trust that management will give them the best possible wages and working conditions unless it is pressured by unions. Of course, not all workers do want to unionize. There are those who worry that union demands will harm the companies and will drive them out of business, thus causing unionization to be counterproductive.