For me it is for items that are identity dependent upon watermarks, color(s), paper type, perforation rates as well as identifying issues such as reperfing, regumming, repairs, cleaned cancels etc.. It also enhances the salability of the item to have a clean fresh cert. The price point at which to start getting certs is a personal decision but I will say that a perfectly centered and fault free $100 cv stamp that could have a market potential of becoming a multi-hundred dollar stamp if graded and certed is worth the effort and cost. In general similar stamps of the same catalog number that I have sold with a certificate realize more then what I realize for the non-certified stamp to the point where the cost of the cert is covered and then some.
One thing to keep in mind is that you are internet bargain hunting and think that you scored that $5000 cv stamp for $250 and you will get it certed and enjoy the windfall you will be sorely disappointed and will be throwing certification money away.
Start with something good in the first place. In stamps more so then most things you get what you pay for.
I see lots of stamps in the marketplace and even shown on this forum occasionally that are misidentified by their owner and it is almost exclusively a case of the stamp actually being a much lower value similar stamp.