Most people are confused by spells. You spurt out a bunch of mumbo jumbo and suddenly stuff happens. There are those who trust in the magic and ones who are confused by it. Why don't you just say "light appear" instead of "lumos"? Well first off, Lumos sounds way cooler. But it's not how it sounds, it's roots from a different language. Most spells come from Latin roots. In Latin "Lux" means light. 
Some other spells have roots in Latin and Greek also. It is like in science the words that have roots in those two languages. Like Aerophobia means fear of flying, Aero means air. 
We'll learn more about this in spells in the next chapter.