Separate amps are not needed for most systems.
The reasons for needing a separate amp....
Really inefficient speakers, or modestly effecient speakers being used in a very large room.
No sub (deep bass has the largest power demands), especially with large full range speakers being used in a large room.
For most of us, maybe 99% of us, a separate amp is not needed. By the way, I own a separate amp (from an older system), and don't use it, my 100w receiver is more than powerful enough for my speakers and room size.