Many, Many things…
Because it is a f1.8, it is faster than a f3.5 or 4.0. Meaning you can take portraits in low light, with a lower ISO rating vs. an f3.5 with a higher ISO rating.
Landscapes: Mornings and evenings..
Action shots, Sports events.. faster lens, better stop motion.
Animals, see above.
It is also a standard lens with most cameras of late, similar to the 50mm 1.7 or 1.4 on the older SLRs.