Let me illustrate with a personal example. When I was first hired as a teacher, I took the place of a teacher who was going on maternity leave. I started in November and finished out the year. I was given a "non-continuing" contract. This meant that I had to resign at the end of the year so that the other teacher could have her job back.
As it turned out, she resigned. I also had to resign so that any teacher with more seniority in the district could bid on the job. Someone did, so I then applied for her job and got it.
At the end of my second year, the district eliminated my position and I was again without a job. Fortunately, a 9th grade position became available and I applied. Another teacher, who had completed his first year with the district, also applied and he had a coaching certificate. He was given the job.
When I asked why he was given the job when I had one more year of experience with the district, they argued that we had the same amount of seniority. Because my first year was with a "non-continuing" contract, they did not count that as a year toward my seniority. I didn't think that was right. Neither did the union leaders.
Needless to say, a grievance was filed and it went to arbitration. The arbitrator, after reading clear contract language that stated seniority was determined by consecutive years of teaching, ruled in my favor. I was given the job.
The district leaders were not particularly happy, but were always kind to me. They were nice men who just really wanted to hire the man with the coaching experience. In order to get what they wanted, they were willing to interpret the master contract in their own way. That is why there are unions ... to protect the workers. I can't imagine what they would have done had they not been nice men. I'm sure my life for the next 32 years could have been very unpleasant.
To do away with collective bargaining leaves workers, salaries, benefits, and working conditions at the whims of their employers. As I said, I'd like to think they would always do the right thing, but they don't. Even the nice ones make mistakes. Imagine what the not-nice ones might do.
And just for your information, the man with the coaching experience also got a job when another teacher moved. The coach and I continued to work together for another 30 years. He understood why the situation ended as it did. He was a union member also.