A systainer takes up more volume than the tool without systainer, but sustainers save space.
I now have 40+ of them. I now have more space than ever and it's really simple. They give you a common size block. Without the systainer you have tools laying around, or have to be put in something. So basically you have random tool piles. Stuff gets lost and scattered. With systainers the tool has a place, plus all the stuff that goes with it. Unlike other manufactures who make cases, festool don't optimize the case to the tool, they pick what it will fit in, and so you have a common stack, plus extra room for stuff that gets added to the kit. You can almost never get stuff back into other tool brand cases as they are custom made for that tool and everything fits in a very particular way. You don't have to be very perfect with the systainers. Since they are their own good storage, you can stack them up anyplace, I have a bunch of the sys-carts and so the tools live on them, most the time the ones I use at that time are on the top of each stack.
All the stuff for the tools are with them, they stay clean. I can pile them up, don't have to make a place to put them. And if I need to go from house to garage, or out in the yard, different floor, or down to a neighbors, just grab the ones I need and I'm out the door.
Once I bought systainers for all my tools it was amazing how much cleaner things were, space savings, organized, and easier to do stuff. I even bought some that seamed silly/over priced like the caulk one. It's great. No more piles of tubes rolling away, or plastic bags with tips poking thru. Everything is in there, included caps for tubes, openers, pokers. It's a one stop place for them and means I no longer have a tube of something I forget about and I buy more of something just to find I had a tube of something and now it's too old. It looks like a waste of space until you use it and you find it is just right.
Sure you can put the tools in other things like cabinets, but now you have to have cabinets, or drawers and those move around less easy and take up their own space. I buy tools now based on ensuring I can find a way to set them up in a systainer.
I'm not a mobile contractor, just a person with a house. But I want stuff to be easily moved, and with a common package. Plus if I get to having cabinets, the systainers can all fit in them just fine, a solid mass of them, and I can almost guarantee it will be packed more dense than not having them while being far easier to get at stuff.
I found it was very easy to get in the habit of putting the tools back in their cases during the course of a project or every night during cleanup. Keeps sanity nicely. Plus now I'm no longer digging all over the place to find a wrench for a tools, or extra blades, and such, it's all right there. A lot of tools that need additional tools like a screw driver or something I just bought one for that tool and put it in the systainer with it, so now no going to find it in a different tool box, anything that tool needs is right there.
I had tools on shelves and tables and scattered all over for a long time, never going back to that. That's not to say I wouldn't mind having a proper shop setup and have cabinets with drawers with systainers on them, but the cases or something like them is staying long term.