HECK no you don't need to know what you're making! Just buy 10 of everything and make sure that you buy 10 of something else that goes with it. Everytime you see a pattern/project that you think you MIGHT want to knit sometime in the next 10 years, buy enough yarn to make in in 3 different sizes in 4 colors. And, most importantly, learn these phrases: "no honey, I had that, remember? I bought it last August!"
"Well, at least I'm not gambling!" and finally "PLEASE! I could buy all this yarn at the yarn store, and pay at least double! I'm saving money!"