Why use Convex Polygons and not Concave ones in path-finding?

I read in Unity’s path-finding documentation that they use convex polygons because there won’t be any ‘obstruction’ between 2 points. Then they add their vertices as nodes along with starting and ending points and traverse them using A* algorithm to reach the required destination.

However, I do not understand what they mean by "no obstruction between 2 points". I tried to check the differences between concave and convex polygons but only the angle differences come up (in convex the interior angles must be less than 180 degrees)