To my knowledge, simulcast sites are done for two reasons: the need for better in-building coverage in larger/denser buildings and/or a shortage of 800Mhz pairs to use in areas that need more sites for coverage.
The majority of the outstate sites are not simulcast, but single or ASR sites (Area Site Repeater??).
For a scanner listener, simulcast sites are often a pain to listen to, especially if the sites are an equal distance away. The cheaper receivers in consumer scanners have a harder time sorting out the effects of multipath interference, where a signal from one site arrives slightly out of phase from that of another site, often canceling itself out. When this happens, your scanner may show full signal strength but have broken, garbled, or no audio.