Since this in posted in the beer brewing equipment discussion, I'll assume you are asking for beer.
During the brew session, the refractometer is a great tool because it doesn't require a large sample, so you can quickly cool a few drops to measure your pre-boil gravity, make checks along the way if you need to, and can get a post boil OG easily. But once fermentation has started, the alcohol will throw the readings off such that you'll need a formula for correction, and while many formulas are out there, at this point speed and temperature are less of an issue, so a hydrometer works much easier.
I personally use just a refractometer as I only do 5-10 L batches so a couple of drops for a sample is very helpful. Aside from a standard hydrometer there are also finishing hydrometers which are designed to just read final gravity accurately.