When working on heavy duty diesels and equipment; the tools are heavier and larger.
The drawers in that box lack the depth to hold many of these tools. Stuffing large tools in sideways can be a hassle, The box is not as stable as a deeper box, either.. They are a far better fit for automobile mechanics.... Why do some Diesel Mechanics have these small boxes? When starting out in a trade, You may not have the funds to buy a new larger box.
So you buy what is affordable, and available, at the time.. You continue to purchase tools, and soon overload that box..
That is a good box for home use, automobile use. or lighter duty chores for anyone else.. But it is still not a heavy duty box..
. I had the mate to that SnapOn box, and gave it to My Youngest Son because it was not suitable for holding the heavy tools I filled it with.. Did I ever mention that I had the bottom of a KRA307F collapse.
SnapOn replaced that Roller cabinet under warranty. Two weeks later, I traded that unused new roller cabinet on a Heavy Duty Road Chest.. I then bought the original roller cabinet back from SnapOn at a salvage price. I repaired it, and took it home. That old KRA307F is built a heck of a lot stronger, and is almost 21" deep, but sadly; only 32" wide.. Way better suited, stronger, yet it still failed for it's intended use. Since 1981 that box has been pressed into lighter shop use three times. It has been holding gear pullers and smaller special tools the last few years, and gets moved very little. That one will eventually be given to My Oldest Son.
I also have two matching sets of Craftsman Griplatch boxes bought new in 2005. They were bought for moderate shop use, and have had zero problems.. Nice boxes, but their 18" depth chops the drawers when compared to a deeper box.. This is a short-******* for all these "skinny" 18" deep boxes... These two sets are now used for organized small parts, supplies, and fastener storage.. They are not overloaded, and are the most attractive setup I have ever used for parts storage.. A fifth cheapo Craftsman roller cabinet was purchased at the same time to hold My Tombstone Welder, and welding supplies.
My SnapOn Road Chest has been in My van for the last few years. It is unreal what it can hold it it's deep drawers. I bought it new but used ones are usually priced at $800 to $1000. The beast is My pet for many reasons, the main reason being it is heavy duty.
56" General? I have a set. Like My Road Chest, I am using it for what I bought it for, and that is holding tools.(?). Can it be overloaded?? I believe so. I found if I loaded a drawer heavy enough with large sockets, the belly of the drawer will drag on the drawer beneath it. However, not every Diesel Mechanic has to furnish their own 3/4" and !" drive sockets, both in hand drive and impact, metric and us, then add deep-well. But even if you have to furnish one set, and a batch of big wrenches they get heavy f a s t..
I chose to keep the large tools in their own steel cases and thus more portable. After all, I do most of my work offsite. The General is the strongest Roller Cabinet of the ones I have mentioned.. Far Stronger than My old KRA307F, which is way far stronger than the mate I had to the OP.
Question for Everybody: Do You actually Enjoy working out of a box that is Overloaded?
Have you ever worried about a drawer slide breaking when opening a drawer?
A new 56" General roller cabinet can be bought for about the same money the OP'S combo can be bought for. It is over 22" deep, and can be used with or without a top chest.
The OP'S combo is a good set for many people, but definitely not all of us.. What more can I say. I tried one as an extra box for semi stationary tool storage.. It didn't work out for Me, so now My Son has a box well suited for a less demanding use
. After adding a loaded chest on top, the weight of those 1" drive sockets also made me question how long the slides would hold. The belly of the bottom drawer was also sagging under that weight... So, it didn't look like, and proved to not be a heavy duty enough cabinet... I gave it to one of My Sons to use for far less demanding tasks.. Then I filled it with tools, but drew the line at 1/2" drive. He did joke about a top chest, as My Road Chest was in the van when I delivered the roller cabinet to His Mom's place.. The plan was to remove the casters and mount it under His workbench.. Only problem is the workbench is at His place near Seattle, and that Roller Cabinet is at His Mothers place in Minnesota.. Well, I'm supposed to deliver it to His house.. 
I do admit that I haven't turned wrenches professionaly in decades so again, current mileage may very. 
(That is why I bolted 2 bottoms together back in late 80's)
