Don't understand why holes needs to be connected to edge These are two holes I cut by using bool tool, my question is why the edges 1,2,3 are necessary, I have a rough idea that all the shape needs to be at least N-gons, so if there is a standalong hole without any connection to the mesh, it is not allowed, but why should we follow this rule, in other words,
1.why the feature is designed in such way, is this reasonable or not, I think this question will help me better understand the mesh, so thanks in advance!
2.Is edge 1,2,3 has to be at the same location as shown on the screen shot? Can the edge 1 replaced by edge a?
There's evidently some critical conditions that an ngon can't exist under.
If you take an example with only one hole, you'll see you are completely unable to dissolve these edges.

When I cut two holes I got a different result from you, and I was unable to dissolve any edges.

Until I added a new edge in the center like you have, and then you are able to dissolve a couple.  I get one less edge than yours.

But each of these remains a shape with a definite inside and outside, and that allows a face to be constructed from it.
I'm unaware of the exact limits placed on ngons, but my best guess is that these are very close to the simplest shapes possible that still allow you to draw triangles without drawing through an area where the mesh shouldn't have a face.

(I kind of misplaced the X here, maybe)
You can use a Triangulate modifier in Wireframe view to visualize.