Some mowers have bolt-on axle stubs so you can put the wheels behind or beside the mower.
As said above, behind lets you mow close to obstructions. But in this position the mower is long and will drag on rises in the ground. If you are mowing rough ground and aren't trying to use the mower to smooth the terrain then setting the wheels beside the mower helps keep the blades out of the dirt because the wheels are at the center of the blades. I set the wheels this way to mow across terraces which are about impossible to mow otherwise. These are old, narrow horse implement terraces.