The initial pour of hot water in pour over brewing to pre-wet grounds and allow CO₂ to escape before the main pour begins. Fresh coffee releases visible CO₂ gas, creating a dome-like swell. Blooming ensures even extraction in the subsequent pour.
If your freshly roasted coffee puffs up dramatically during bloom, that's CO₂ escaping — good sign of fresh beans. Older coffee blooms less vigorously.