I'm pretty sure it's happened to you, because it's happened to me enough times: you find a knitting pattern you like only to discover it's not quite what you want or won't quite meet your needs. Maybe you need it to be a little bigger or smaller. Maybe you're using a different yarn and can't get gauge no matter how hard you try.
I was confronted with this problem recently when I wanted to knit a super-simple Stockinette hoodie for the girl. Now I could just write a pattern for a hoodie, lickety split, but that wasn't really the point. This one had numbers already! The only problem was, it was worked in a bulky yarn and I didn't have that kind of yardage. So I held two medium-weight strands together. It's still a bulky look, but not quite the bulky gauge the pattern called for (I got 11 stitches per 4 inches instead of 8).
Clearly a little altering needed to be done. In this case, it was just a teensy bit of simple math, but when it comes to basic patterns, it's pretty easy to do minor alterations to the pattern like this. You do have to know your gauge, of course, the measurements you're trying to reach and where the numbers change in the pattern you're knitting so you can adjust accordingly, but once you've done it and successfully knit a project you altered in this way your knitting (and design) confidence will increase exponentially.