A roller compared with a dropper bottle
Both put serum on your face. The difference is how much of it gets there and how evenly.
What happens with a dropper
Serum goes into your palm, you rub your hands together, and a real share of it stays on your hands. Coverage depends on how carefully you spread it, which varies with how tired you are.
What happens with a roller
The serum comes through the head as it moves, so it goes onto the skin rather than into your palm, and coverage is as even as the path you rolled. Nothing is soaked up on the way.
When a dropper is fine
If you are using a heavy oil or a thick treatment, a roller is the wrong tool. Rollers suit light, watery serums, which is what this one is.