After fermentation, the refractometer won't work. People use calculators to compensate, but none of them are particularly accurate- they're all basically just guesses at about how much alcohol has been produced. Use a hydrometer for post-pitch gravities.
The reason to get one for pre-pitch gravities is 1)You don't need to pull a big sample and 2) You can measure gravity at every point in the brew day to make adjustments on the fly. You can get gravities during and after the mash to see if you need to go longer, you can get your pre-boil gravity to make sure you have all the sugar you need, in case you want to add DME or something, and you can check how the boil is progressing (boil-off is where it usually is) as well as taking your final pre-pitch OG. It's more of a tool for advanced all-grain brewers.