Let's face it. Knitting kits make projects a lot easier to begin.
In the past I would discover a pattern that made my heart sing and want to create that exact look. I’d buy the pattern and then start searching for the suggested yarn. More often than not, it was something special and not carried in one of the large craft stores. So I would search either online or in one of the local yarn stores. (I live in Southern California, so if I'm willing to brave a fair amount of traffic, I have a few choices.)
Someone would suggest a fiber that wasn’t quite what the pattern was knit in, but "should" work and I'd go for it, with the picture of what I really wanted dancing in my head.
So often I ended up disappointed with the results. Sometimes it turned out great, but more often than not it wasn’t quite right in sizing or how it draped. So there I'd be with a bunch of time, not to mention money, invested in creating something that was just okay.
I’m a much more experienced knitter now and better at knowing myself what might substitute well and how to swatch a gauge properly to make sure it will knit up in the size I want. But that takes years of practice and a tolerance to be wrong sometimes.
If it’s in a kit you can be sure that this pattern and this yarn are a match made in heaven. Someone else has already done the trial and error of discovering the sizing and how the fabric will drape.
Also, if you need only a few yards of a color, a kit will include (and charge you for) only what you need instead buying an entire skein with most of it ending up in your private yarn vault. (Don’t get me wrong, I treasure my yarn vault. But even I know there’s a limit… isn't there?)
I agree that some of the pleasures of knitting are the surprises that happen when different yarns and patterns are put together. And stepping out into the unknown, adjusting the pattern along the way can be an exhilarating way to create.
However I believe the ease and surety of seeing what you want and knowing you have all the elements to make it are a great way to learn new techniques and ultimately be ecstatic with your finished project.