With a rack sampler, you need to supply a sequencer (MPC or other) and a MIDI controller (MPC pads, MIDI keyboard or both) to use it.
The only time you really need a rack sampler is if your MPC doesn't meet your sampling needs or if you don't like the MPC sound quality, workflow, or if it doesn't have the sampling features and capabilities you need etc... Or if you want a sound module that has a customizable soundset...
Another option is to use a software sampler. and the above applies here as well.
Also, you have to position it so that you can reach the controls comfortably, because you'll spend a lot of time pushing buttons and twisting knobs on it.
how does sampling work on those joints?
The concepts are the same as sampling on your MPC or ASR or any other sampler. How it functions exactly depends on which sampler you're talking about. Same goes for editing, mapping, etc... Some have more, less, or different features than others. And they all have different workflows.
They're so cheap now that you can get one, play with it, decide if you like it or not, then sell it and get a different one. Check out harmony-central.com and sonicstate.com for user reviews.