Its sole purpose is there to make it longer, even if it can be canceled out of.Bosses being able to self heal is just as bad a gimmick as adds in a boss fight. And I wish it was just the random stuns (when she starts gushing blood) on the charge hits. The hitboxes on her overhead swing are random (sometimes getting hit on the same dodge), her moveset is boring and repetitive, the occasional tracking on her jump is way too much to where she starts turning direction mid jump. This feels like a DS2 boss more than a BB boss.
No, that's not its sole purpose. Sorry to say that you are just bad at that fight and you're blaming the design for your own shortcomings. And the healing doesn't even extend the fightyou can use it to your advantage and do more damage than the heal.