I almost never use the yarn given in a pattern.
Yarn sub is great if you want to try to get as close as possible to the type of yarn you are substituting. You can change it up on there too though, say if the yarn is acrylic but you want natural fibres or if the yarn is wool but you prefer man-made options, there is an amount of flexibility to how you search - including budget.
I don’t like fluffy yarn with a haze it tickles my nose and makes my face itchy but if I liked a pattern I’d make it in a non fluffy yarn instead. This is when yarn subnis not quite so useful to me as it will try to match the fluff and haze of the original yarn when I want something quite different. Hope that makes sense.
I have used yarn sub but I’ve also just chosen my own yarn for a pattern. Obviously you need to accept the garment won’t be exactly as the pattern if you change the yarn.
Usually I find a yarn I want to use (my own decisions have been based on fibre, recycled yarns, cost, colour, ease of washing, for instance) and put it with a pattern with the same weight. Mostly DK yarn and a DK pattern. Then I swatch to make sure I like the fabric and see how the gauge compares to the pattern.
Usually my gauge comes up as given in the pattern with the needle size in the pattern and I go ahead.
I had a DK yarn recently which I got gauge on for a sweater, same gauge as on both the yarn band and the pattern - worked out well.
Then i used the same yarn for a different pattern, still DK but the pattern called for a smaller needle. I didn’t like the swatch at all it was tight and stiff and I thought I might have to ditch the pattern but after some thought I decided I should use the needle size on the yarn band to produce a nice fabric (no need to swatch again as I’d already used it on a sweater and liked the resulting fabric) and alter the pattern size if needed. A bit of extra maths but not impossible.
I like the freedom to choose my own yarn and pattern. It doesn’t always work out how I imagine though. I chose a recycled yarn (for the reason that it was recycled) for a lace sweater pattern and when I swatched the lace didn’t look good at all in that yarn, the yarn was too tweedy/mottled. I just found a different pattern for that yarn and made a cable cardigan with it instead.
Picking your own yarn requires a bit of maths to work out the meterage you need to purchase. You can’t just go ahead and buy 6 balls because the pattern says so.
Hope this helps and I hope you enjoy finding some yarn.