I would say that they can do both and have done both over the years. Everything depends upon the people leading management and the union. Some folks often seem eager for conflict and intransigence. Some people want to earn reputations as "tough negotiators." Much depends on the economic situation at the time when any particular negotiations take place; unions are less likely to be intransigent when economic conditions are bleak; management is less likely to be intransigent when the labor market is tight.
I'd say that depends on the union and the management as well as the industry that is under review. I was part of the teacher's union and I think it really helped to avoid conflict between the teachers and the county officials. Unions can provide a lot of benefits for participants as well. Sometimes these benefits can take some of the pressure off of the management. A union might make negotiations easier because the management has a single entity to deal with rather than thousands of individuals. I think unions have their purpose but I can certainly see the negative aspects of them as well. One has only to look at some of the recent news about sports unions to see how greater conflicts and stalemates can arise.
If anyone has looked into the history of labor before the organization of unions, he/she will realize how they came to be--just reading Upton Sinclair's The Jungle is sufficient for many. However, the problem nowadays is the problem of human nature: Whenever people become too empowered they become unreasonable. Major labor unions such as the Teamsters, the UAW (auto workers), the steelworkers, and others have often priced themselves out of jobs as the union members, who are often too short-sighted, vote for increased wages without considering that their company is not making enough profits to afford the increase, or without realizing that the consumer of their product makes much less than they do so that consumer cannot possibly afford their product. Because such workers are short-sighted and selfish, they are certainly the cause of problems for management and the success of a company. This conflict is the greatest one today.
Recent documentaries on the autoworkers' unions have demonstrated on tape that GM employees are sometimes paid without even doing any work; other ridiculous conditions exist, too, such as the employees' smoking marijuana at lunchtime or drinking without any repercussions. In other words, the union is so strong that it is virtually impossible for a company to fire unworthy employees. Obviously, the pendulum has swung too far in the other direction from the days in which the owners of New York textile factories were able to throw their Irish shopgirls into the machinery if they did not work long hours and suffer the horrible conditions dealt them.
Typically, I'd say they make it worse. I'm not saying that they don't do the workers a lot of good, but I do think that having an organization whose whole purpose (in a sense) is to argue with management does tend to increase the degree of conflict.