A ground motion record (accelerogram) is a measurement with high specificity to the fault that caused the earthquake. The recorded peak acceleration, frequencies, duration etc depend on the fault geometry and movement. Of course there are not two identical faults and every time a fault is activated it doesn't produce the same ground motion. Therefore, the value of designing a structure by one "as recorded" ground motion is small, since it is highly unlikely to encounter it in the future. Typically the design with real ground motions requires a set of them (at least 5).
Why scale a ground motion
In an ideal world a structural engineer would have a vast amount of historical ground motions to select from, for a particular site, when designing a new structure. There would be no need to use any records from other parts of the world. Of course this is not happening because severe earthquakes are not an every-day phenomenon and recording ground accelerations started not to far in the past. There are sites where only one or two good records are available. So the solution is to use available records from all over the world, but then the problem is that the seismic risk is not the same all over the planet. Typically, the seismic code provides us with the expected intensity of the seismic excitation, in the form of peak ground acceleration. A solution is to scale an accelerogram to match the code requirements. A problem arises though, because a distrortion in the frequencies is unavoidable. Therefore, it is better to avoid too much scaling, or alternativly to use artificial accelerograms.
Also, scaling of the ground motions is inherently needed for some advanced types of analyses (incremental dynamic analysis).
