There is an excellent video on Autodesk website for this..
What 2.5d snap does, exemple: Go on a TOP view, you have a rectangle on the floor and an other rectangle with a Z elevation of 50 units. With the 2.5d snap activated, always in a top view, you will move the second rectangle and snap it to the rectangle laying on the ground level.
From a top view, they are aligned, but if you swivel the view a little, in a user or perspective view, you will se that they are not at the same height.. so they did "snap" on the X and Y axis, but not on the Z axis.
This tool is usefull to align objects that are not on the same plane. (Align objects, in a row)
Always use this tool from a flat view ( Top, bottom, left, right, front) but not from a user or perspective view