Lets say you have two poly cylinders that are NOT intersected and you combine them. Now when you select one, the other is also selected because they are part of the same shape node. However, you can still plainly see two separate cylinders, so there are two separate meshes. In a single mesh, everthing runs together, there is no separation like that.

Boolean will omit the faces that are no longer needed. i.e. those inside the union area so that the result should resemble two objects welded together to from a single surface area.