Can anyone tell me why the extra horsepower would be needed?

Short answer, No.

As others have said there are some scenarios where it can be very handy. To
mitigate the problems associated with a marginally underpowered saw: you can
do the following:

1. Tune your saw well
2. Use a sharp blade
3. Use a thin kerf blade
4. Use the right blade type: e.g., rip blade, with a lower tooth count
5. For a really big cut, take multiple passes.

Just about every here has owned/used a 1.5 HP contractors saw at some point
and we all know they can produce fine work.

-Steve