Why? And why is Doffy relevant?
Firstly from what we have seen and been told the current top 5 (outside of Luffy & Imu) is Shanks, Mihawk, Dragon, Akainu and BB. That is likely a level just beyond whatever Ray reached. I would put all 5 of them at 4.5-6B when they are ready to really go to war.
Shanks has been wanked left and right by everybody as a PK-level character who for unknown reasons sacrificed his arm, is beyond the Gorosei/Marines reach, decided to keep the peace and refused to ascend to PK.
You could say all of the top 5 have bided their time until now which meant their true strength are yet to be reflected in their bounties but now the time is right for creating havoc.
The comparison between Shanks and Law (Doffy) is that a top level pirate directly took in a baby/kid into their crews and that the Captain + the rest of the crew taught/inspired those kids. The crews themselves believed that Shanks/Law would become at worst right hand man of the captain or at best actually ascend to the very summit after the captain retires/steps aside or whatever.