An online estimate is a computer model. It reads whatever public information it can reach and returns one confident figure. It has never stood in your kitchen, does not know the furnace was replaced, cannot tell a ravine lot from one backing onto a parking lot, and has no idea which buyers walked away or why.
A comparative market analysis is done by a licensed person with access to the sold data. I pull homes genuinely comparable to yours — same pocket, similar size, age and condition — that actually closed, not ones merely listed and hoping. A list price is an ask. A sold price is an agreement two parties reached.
Then I adjust between those sales and your home for what differs: a finished basement, a garage, exposure, what has been renovated and what has not. The result is a supported range with the reasoning attached, so you can push back on any part of it. You can argue with a comparative market analysis. There is no arguing with an algorithm.