That's easy to answer: there's all kinds of things that you might want to do which can't be done exactly right with a commercial product. With an Arduino, you

That's easy to answer: there's all kinds of things that you might want to do which can't be done exactly right with a commercial product. With an Arduino, you can program it to do exactly what you want. If it's an extremely small niche application and there is a commercially-available product, it's likely far more expensive than the Arduino because of supply and demand. And it probably still doesn't do exactly what you want.