Usually, when workers were had bad working conditions, they went on strike. Most were scared to go on strike because they were arrested and were fired from their job. Therefore, many decided to join together and form unions because this way, they could not be fired because unions contained a huge amount of people. If all these people were fired, many owners would run out of business.
To join these unions, many worked together to get better working conditions and rights. They went on strike and went against poor working conditions. They wanted higher wages.
To sum this up: Many joined unions so that they may not be fired from their job while still being able to strike for better working conditions.
Hope I helped. :)