Deads strengthen the "posterior chain," from neck to the soles of your feet. We tend to view deads as a legs exertion, but no, not only. That's why they are an invaluable totaly body exertion.
That posterior chain of muscles is what it makes it possible to be bipedal and to stand up straight instead of slouching. It's counterbalanced by various anterior muscle groups, expecially the abs.
I am no body-builder guy, but after 4 years of including deads in my routine twice weekly (pushing the weights, admittedly, but gradually with high-rep days and low rep days), I can no longer button my dress shirts. 2" more. That's crazy, not what I wanted but it just happened. Brooks Brothers shirts ain't cheap, and I fairly often wear jacket and tie at work, especially for meetings when it is expected. I don't have a single dress shirt that I can button now.
Deads are the single best way, and most efficient way, to stay sturdy after age 40.