Janitors keep places clean so programmers can focus on programming and thus "contribute to society." Assuming programmers are doing some good, not cleaning their own office lets them do more of it.

In this way I think janitors can contribute significantly. But I think the more important point is that contribution to society is always a team effort, not an individual thing. Any contribution you make is in part thanks to your parents, teachers, co-workers, janitors, and so on. They deserve some credit.