A RepRap can print plastic objects (PLA, ABS, HDPE…), with dimensions up to 200x200x140mm.
You can print a lot of common objects, born of necessity or fantasy: key-ring, pen holder, knob, model, toys…
Remember that with a RepRap you can print the plastic parts to build another RepRap.
But RepRap has obviously some limitations. The objects must remain under a certain dimension; you can print a big object of course, but piece by piece, and maybe they won’t fit each other perfectly.
The print quality isn’t extraordinary (it depends on your experience) and the surface finish appears rough.
It’s not completely replicable: several parts of the printer aren’t in plastic and aren’t printable.