Nothing much to add to what Sabby says.
Skimmers don't really work well below SG 1.010 (though some designs might do better than others). So if you're keeping a low-end tank at SG 1.003 or 1.005, then you will probably not find a skimmer useful.
Skimmers can be expensive. But the other way of looking at them is this: because they remove organic materials directly, before those materials are converted into nitrate, you can keep nitrate levels lower with fewer water changes. So this can be one way to reduce the cost of your tank in the long term.
They probably make more of a difference with high metabolism fish (like scats, monos and puffers) than they do with fish that mostly sit about doing nothing, like morays, gobies and scorpionfish.
Cheers, Neale