In the picture above is a Samurai Differential with 3.72 gears. To the left is a typical Differential "can". Below on the left is a Tracker 4.625 ring gear out of the front of a TracKick. On the right is a stock 3.72 Samurai ring gear.
Here is a close-up of the ring gears. Note that the vertical face of the inside edge of the 4.62 gear on the left is shorter than on the 3.72 on the right. This is where the "can" comes in...
See the pins in the picture of the stock samurai differential above? Note that the pin in the upper part of the picture is nearly flush with the outer surface of the center section while the lower pin is sticking out a bit because of gravity. If you were to install a 4.62 ring gear in the place of the 3.72, that pin would slide out much further causing the spider gear to slip out of place resulting in lots of bad grinding and breaking activity!
The differential "can" is installed to take the place of the missing part of the 4.62 ring gear inner edge (remember, the 4.62 is not designed to to perfectly replace the 3.72..) so the pins do not fall out of place.