OK. Do you know the PLL that uses a XOR as phase compare function ?. OK. you then also know that the frequency that is coming out of this XOR is your reference frequency let say 1Mhz, then you also have 1 MHz frequency steps !. OK ?. Now your PLL filter needs to filter this frequency otherwise you will have next to your carrier left and right at 1 MHz offset a signal.
The problem with this type of PLL is that your loop bandthwith and your reference frequency are always liked. Large loop bandwith is nice since you can lock fast and suppress the inband noise of your PLL but your stepsize is always large since your loop bandwith is large. That is why they designed fractional N (to take a frequency step wich is a fraction of your reference frequency). This PLL works nice but has many surious signals we do not want making your loop filter complex and still not perfect. If we take the fractional N and increase the frequency but leave the duty cycle in tact you get a working PLL but your spurious signals are transformed to a higher frequency and thus more easy to filter...