Matching Audio Amplifier with Speaker (Watts Rating/Output)
Amplifiers have more of a hard maximum limit whereas speakers are softer. 

The power limit of an amplifier is basically frequency independent, more or less. It's determined by the voltage at which it clips. When given sine waves, it clips at the same voltage at 100 Hz as at 10,000 Hz (if it is any good, anyway). It also clips any other material at the same voltage. The calculations from that to RMS values are straightforward.

With speakers, what the maximum rating means is not quite so clearly defined.

If you naively match a 50W amplifier to a 50W speaker, you may not be able to explore what that speaker can "really do". Perhaps the speaker can handle being over-powered for short periods of time. Maybe the speaker is okay with 60W for five minutes, and the distortion is not even bad. Or maybe it can handle large transients that occur in material whose average power is low.

In a nutshell, a power amplifier's headroom is very sharply and precisely limited by its power rating (with regard to a given load impedance), whereas that of a speaker not necessarily so.

So it makes sense for an amplifier to be overrated with respect to a speaker rather than vice versa, not just from the perspective of avoiding clipped signals going to speakers, but from the point of view of headroom.

Another regard in which it is good for an amplifier to be over-specified is load handling. An amplifier that can drive 2 ohm loads is more robust than one which can drive only 4 or 8 ohm loads. It's not necessary to have such an amplifier for 8 ohm loads, but the beef doesn't hurt. You never know: a speaker voice coil could melt in such a way that it suddenly has less impedance. Someone could connect too many speakers in parallel. Etc.