Secondary is optional for most beers, besides beers with additives like fruit/hops, and beers that are huge and will sit for several months like a barleywine. Pros and cons, possibly clearer beer and this is debatable, and the risk of adding infection and or oxydation. The pros dont outweigh the cons to most. One of the old school thoughts were to get the beer off the yeast asap to stop autolysis or the yeast dying off from adding unwanted flavors. This has kind of been debunked, it takes longer then your average beer is fermented for this to happen. From my understanding this does happen to wines if left on the lees for months