This can be explained by taking a 3 x 5 card and bending the ends at 90 degrees about 1" in. Kinda looks like a "C"clamp, but it has no strength against racking on the long dimension. A simple fix would be to add a stiffener down the center under the top and attached to the ends. so now we have "planes" of material in 3 axis, X,Y, and Z.
Depending on the strength of the attachments from one plane to the other, you mockup will be relatively stable relative to the thickness of the 3 x 5 card. Within each plane there are hidden diagonal or triangles and you could just as well use toothpicks to illustrate them and how they strengthen the structure.
Depending on the design of the structure you can use the joinery, rather than separate attachments, like metal braces or screws. Mortises and tenons with shoulders and wedges can make a very stable structure, without screws.
A bench will have greater forces applied to it laterally than a table because there may be more than one person sitting on it and moving in opposite or parallel directions with greater force than would normally be on a table.
Then there is the "design" part of the design... HUH?
How does it look? All the diagonal braces get real busy for me and I don't care for the look, even though it may be structurally sound. The simpler the better is my design vocabulary, less is more.