Right. You want your amp to be able to play all your music, loud and soft, without distortion, and be able to handle sudden changes in volume easily. More power gives you just that. It's something that we call "headroom." As a starting point consider the RMS power ratings of your speakers or subs. Match or better yet, exceed the speaker power ratings with your amplifier. For example, to get that headroom, if you have a subwoofer that handles 200-watts RMS, power it with an amp that puts out 250-watts RMS. It'll drive it cleanly and without distortion, especially when the volume is cranked and it'll do it better than an amp with less power. More power is always better. But we haven't talked about one of the most vital factors in amp performance: amp wiring. Your amp depends on a steady supply of power to operate at its peak and that's why you should invest in a high quality amp wiring kit when you buy the amp. If you use too thin or cheap wiring to hook up your amp, you're starving it of the power it needs to do its work. Yeah, the bottom line is you're not going to get the performance that you paid for.
Cheers,
Pan
