Question

What is the disadvantage in comparing line segments by mere observation?

Answer 1

There may be a chance of error due to improper viewing. Improper viewing may be the result of looking at both line segments from different angles or using improper approximations.
We should compare two line segments using a ruler or a scale with accurate measurements.

For example: In the given figure, one cannot compare which line segment is longer, whether $AB$ or $CD$ without having the measurements.