Not to side step the "hardest wood" question, but as far as application... hardwoods are generally used in places where you want strength, but not weight. Things like tool handles, sports equipment (baseball bats, and hockey sticks for example), furniture, and so on.
A cubic foot of steel is incredibly strong, but it's also incredibly heavy at 490 lbs (7900 kg/m3). A cubic foot of hardwood is usually closer to 50 lbs (800 kg/m3).
As far as "the best material for the job" I've always had a preference for wood hockey sticks, but wood wrapped in Kevlar helps with wear and tear from... Well... Hockey. They're lightweight, slightly flexible, but still incredibly strong. My first stick is more than 20 years old and it's still usable despite some dings from rougher plays.
As far as construction goes... Of course steel and concrete are stronger and in many cases more durable, but they're much heavier and they're also much more expensive.
Also... The hardest wood possible really depends on the application and whether you're talking about a simple slab or a composite ply.
