why? cuz you're getting more for the price and it gives a satisfaction to know that you're getting what you need and a lil more.
But in some cases, you might reach full excursion or power a sub to it's full potential. Another thing is that an underrated amp might run cooler or more efficiently than others. IMO
Watt ratings are kind of subjective, some amps put out and others lie. RMS is the only figure worth reading. On top of that they rate at different voltages. And the subs, enclosure makes a lot of difference how much they can handle.
And my gains proper are the most I can get...but yes it could be limited by speakers or amp.
It is not just car stereo, once I had a 100Wx2 pioneer home amp. I got a hold of an equally old harmon kardon 60x2 that would blow the pioneer out of the room. The HK is underrated, they still are today.