Gearbox Software's Randy Pitchford has made it very apparent why developers would spend time building Achievements into their games.

"The time it takes is minimal because you're designing Achievements anyway, and you can probably affect your sales by something like 10 and 40 thousand units. If you're talking about a triple-A game selling between 1 and 2.5 million units. You're talking tens of thousands of units of impact there."

When he says "Affect your sales" in this interview with OXM, he means "increase" - because why else would you spend any time - no matter how minimal - making them. Still, 40k sales, eh?