The major benefit of the UAW, like that of other major unions, was that it helped to ensure that its members enjoyed good working conditions and good salaries. By doing this, the UAW helped to create a strong middle class that included even blue collar workers.
In the heyday of the American auto industry, there was very little competition for American companies. They were able to make very solid profits on a regular basis. Therefore, they were also willing to give rather lucrative contracts to their unions in exchange for labor peace. This was the major benefit brought by the UAW. It negotiated these contracts that paid very good wages to people who had very little education. In the post-WWII era, it was possible for people who were in the UAW to earn wages that put them in the middle class even if they had no education past high school. This was materially very beneficial to the members of the UAW. It was also, one could argue, very good for our society as a whole because it provided economic opportunity even to people who lacked the aptitude and/or the desire to go to college.
Thus, the UAW was helpful both to its members and, arguably, to society as a whole.