People's vision is limited, and intuitive judgment only by eyes sometimes leads us to draw wrong conclusions that are inconsistent with the facts.
Please look at the following example:
(1) Which of the two arcs in the diagram 1 is longer? It seems that the lower arc is longer than the upper arc, but it is actually the same length.
(2) Which square do you think is in Figure 2? Looks like the one on the left? One is a square. Actually, you can know the one on the right by measuring it? One is a square.
Figure 1
Figure 2
(3) In the parallelogram of Figure 3, which line segment is longer, AE or BE? In fact, AE is as long as BE.
(4) In Figure 4, AB, CD, EF and GH are four parallel lines, but they don't look parallel, which is the result of diagonal background interference.
Figure 3
Figure 4
So in the geometric world, intuition is often unreliable, so it needs to be verified in various ways.