They're for people who want to spend some intimate time and cut out the cards themselves. Just kidding.
An uncut sheet is essentially what the name says they are: a sheet of 56 cards that has been pulled out of the production process before they are cut into strips and eventually singular cards. They are usually more valuable than the respective deck, though much more difficult to sell as demand is incredibly limited. Most people purchase them for their own collection, or to display it as it is/inside a frame (Which i never really understood why.. it's insanely unattractive in my opinion and i would much rather fill my wall up with classy aesthetically pleasing posters/artwork rather than repeated pip designs, but that's just me). As for quality, most of the uncut sheets you will encounter are from USPCC, thus quality is more or less identical. Just imagine a big sheet of bicycle/bee/aristocrat stock with a few variations in finish. Sometimes (especially small kickstarter projects), you may find uncut sheets that have not even been dimpled/lack a finish. These are even more 'rare' and priced ridiculously for those crazy collectors out there.
tl;dr- uncut shits r stubid. onli buy if u want 4 collecsion. do naht bye 4 resail. uspcc qualitie gud, littal diffarince.