The laws of physics are same in all inertial frames of references. In fact, if it we find a "law" which is not so, we believe it is not a fundamental law.
To find the importance of an assumption, remove it and see the consequences. If the laws were different in distinct reference frames, that would be a terrible world to live in. Nobody would agree upon a measurement since they all could be potentially observing different things.
Just to give an example, suppose you are in a bus and you throw a ball upwards. If the bus is at rest, obviously the ball will fall straight down to your foot. But what if the bus is moving forwards at a constant speed (that is, “so long as the motion is uniform and not fluctuating this way and that”)?
One’s first guess might be that the ball will fall somewhere behind the foot or might as well hit the back of the bus. But it turns out not to be so; the ball will again fall at the foot. Had the laws been different, there would be no guarantee that the ball would fall back. But, of course that is not true.
In particular, this experimental verification of first law gives us enough confidence to assume the laws of physics to be same in all inertial frames. Since theory of relativity works well in explaining dynamics at higher velocities and subsumes to Newtonian regime as a low-velocity limit, this assumption is important and necessary. We have a strong reason to believe so.
In the end, these assumptions are tested time and again and you can find many useful references here.