Well, they used it because it's a simple IC, just 7 darlington transistors , with resistors on the bases of each transistor (on the UL chips ending in A). It saves PCB space as you don't need to use additional resistors.
There's no 5v, the transistors inside can work with up to 30v or something like that, check the datasheet to be sure.
There's several versions in the UL2* series, the differences being in the maximum voltages the individual transistors can handle and the value of the base resistors (and if there are base resistors built in or not) ... depending on the base resistors built in, some UL2* series chips will work with 5v signals on the base pins, some would need at least 8-10v or something like that.
There's also UL2803a series which has 8 darlington npn transistors .. and there's also chips which have pnp transistors but don't know the names.