As someone who flies a lot as part of my job, I mostly use airline miles to upgrade my seat to business class. On a few occasions I cover the upgrade myself if the price is reasonable.

Why? Because as a somewhat tall guy, when I have a 10 hour flight a coach seat makes me ache for days. And if it's an overnight flight, a business class seat actually enables me to sleep.

On business trips that only last a couple of days, the project managers sometimes approves business class tickets, so that we're actually fit for work when arriving.

Any flights longer than 5-6 hours is usually when I begin to consider upgrading. Because then I have a chance to actually get some rest onboard instead of just sitting there, waiting.

EDIT: Business class comes with lounge access, and for some that's also a plus, but my frequent flier level already gives me lounge access all over the world, even when flying riffraff-class. Plus business class also comes with extra luggage allowance, but I have that by default too.