It started when I wanted to make my own taco seasoning, which called for both these ingredients. And what's the point of growing all these herbs and veggies but still buying their powders? Google is a good friend, I tell ya. I come up with a question and it provides me an answer every time.
The most obvious benefits to making homemade garlic or onion powder is the cost: With onions or garlic purchased from the farmer's market I'm paying about half the price I would pay for powder at Trader Joe's. And if I ever manage to get my own garlic and onions to grow, even less. Also, the flavor is so much stronger than store-bought powders, so I use a bit less. And on top of that, there are no additives like "Anti-caking agents", whatever those may be. And drying can be done numerous ways, from free to next to nothing. Dried garlic is obviously not as preferred as fresh, but it comes in handy.