What is the biggest benefit of socialism?
One way to answer this is to say that the biggest benefit of socialism is that it reduces the amount of inequality that exists between members of a society. You can argue that this makes a socialist society more equal and fair than a more capitalist society.
In a capitalist economy like our own, some people have much more money and more life opportunities than others. The kids of a rich person, for example, are much more likely to attend a good public school than the kids of a poor person. This has nothing to do with the talents of the kids and so it is, you can argue, unfair.
Socialist societies should not have problems like this and so you can argue that they are fairer to everyone. You can clearly argue that this is a major benefit.