Being absolute, on power-on the position is known without having to move back to a reference position, as would be required with incremental encoders. Sample rates can be much lower than required with quadrature incremental encoders. As it is absolute, you only have to power the encoder when you want to read the position.
Bourns give more info.
My copy of the look-up table for the PIC for those who have wads of code space spare. Try Version 5 takes up 63 bytes, and requires upto 300 cycles to convert.