In a person with hyperthyroidism, what is the purpose of block replace treatment (giving carbimazole+thyroxine simultaneously) rather than titration?

Anti thyroid therapy. The strategy of suppression and replacement addresses and largely eliminates the frequent dose titrations required by drugs like carbimazole. The downside of this strategy is that it requires higher doses of the anti thyroid medication.
Some doctor's do thi. The vast majority of endocrinologists (myself included) do not utilize this treatment. A few do. I do not see value in it's use, but if your physician does, you should discuss it with him/her.