What's the big deal about a gauge?

What exactly is the gauge? What’s the big deal? Oh, and another rando question, if a pattern mentions a specific type of yarn, e.g. Mountain Colors Winter Lace: red willow, 1 skein, do you have to use that specific yarn for the pattern, or will it work fine with something of similar thickness and material, and going off of that, does what the yarn is made out of matter in a pattern, if it isn’t specifically stated?
Answers to any of these will be much appreciated.
Thanks

What exactly is the gauge? What’s the big deal?

http://www.knittinghelp.com/videos/learn-to-knit

Scroll down and down and you’ll find an explanation of gauge and some videos about it.

Gauge will effect the size of the finished item. A sweater made per instructions but with a different gauge will be a different size and could be much larger or smaller than the size the pattern is supposed to make. Sometimes, it’s not as important. A scarf or afghan doesn’t have to be a specific size to be useful and acceptable.

if a pattern mentions a specific type of yarn, e.g. Mountain Colors Winter Lace: red willow, 1 skein, do you have to use that specific yarn for the pattern, or will it work fine with something of similar thickness and material, and going off of that, does what the yarn is made out of matter in a pattern, if it isn’t specifically stated?

Different yarns produce different results. If you use a yarn dissimilar to the one specified your finished piece will be different. Substitution is OK as long as the yarn is similar or you know your result can be quite different. A pattern that calls for wool will work up quite differently if made in, say, a cotton yarn.

Some people can’t use wool or choose not to. Other can’t or choose not to use synthetics. Sometimes the yarn specified is too expensive or for some other reason a substitution is desirable.

GG has it right. Gauge can be extremely important, but it depends on what your making. You should swatch when making garments for sure.

Yarns can make a great difference if a pattern calls for a lace weight and you use worsted it will not look the same. As long as you understand that it’s not a problem.

If you’re knitting a pattern and can’t wear wool then you look to the gauge to figure out what weight yarn you need to sub. I often use a different yarn.