Mainly it's the ability to absorb energy and punishment. Many materials have an elastic range, where if you deform it within that range, it will return to its o

Mainly it's the ability to absorb energy and punishment. Many materials have an elastic range, where if you deform it within that range, it will return to its original shape. Of those, a lot of materials, including many metals, are relatively brittle - once you push it past its "elastic limit" it just breaks. There are ways to make steel more like that too, but in general steel doesn't break at that point, it "yields" or deforms permanently (inelastically). You can keep on deforming it (and in fact it actually gets slightly stronger while you're doing that, which is an interesting feature) and it goes way way beyond what you would think possible before it finally breaks. So as a result it can absorb tons of energy, which makes it interesting for strength applications. The steel frame of a building in an earthquake for 30 seconds is absorbing tons of energy while hopefully not collapsing, or even in the worst case it at least allows a bunch of extra time for at least some of the occupants to escape. Or a steel-framed car that crashes into a pole - the steel crumples, absorbs energy, and slows the car somewhat more gradually in the process. If the frame were a brittle material it might just shatter on impact.