Merit and meaning of fractional bandwidth
"Still, I could not find a good explanation why I would use the fractional bandwidth to describe for example a filter instead of absolute. "

Well I think, using the fractional bandwidth instead of the absolute bandwidth makes sense, because it is not sufficient to state that the (absolute) bandwidth of a bandpass filter is 1kHz (for example). Thus, to get a feeling about the technical requirements to realize such a filter it is important to know if it is centered at 5 kHz or 5Mhz.
However, there is no principle difference if we are using the absolute bandwidth to describe the sharpness (selectivity) of the bandpass filter or the quality factor Q which is the inverse value. But I think, in most cases (filter tables or filter design programs) we are using the Q value (which also appears in most transfer function forms). 
Remember: For a bandpass, the Q value (selectivity) is identical to the pole Q which is defined based on the pole location in the s-plane. Thus we have the same principle to describe transfer functions for lowpass, highpass and bandpass based on the pole location.