Let me explain.
I am currently designing a lace shawl in the Shetland tradition, with a centre panel, an Old Shale border, and a lace edging. The centre panel is worked first. Stitches are picked up from around the edge and the Old Shale is worked out from it. Then a lace edging is knitted on. Of course, all those require a certain number of stitches or rows for each repeat, and they all need to work together.
I had based my initial calculations on the size of a previous shawl I had made using the same yarn but using a much less open stitch pattern. When I blocked my swatch I noticed how much more I loved the lace when it was blocked really aggressively, but that created an issue. If I blocked it until it begged for mercy, the finished work was going to be huge. I want a shawl I can wear, not something I can lend to boy scouts for camping trips!
Bring on the maths. At school I never would have dreamt that one day I would be trying to calculate the number of repeats required for a 14-row repeat to allow me to pick up stitches around the edge that will work for an 18-stitch Old Shale repeat. But wait, it gets more complicated. That number also needs to be divisible by 12, as my lace edging has a 12-row repeat. And all of it needs to give a shawl with pleasing dimensions.
"Never going to need maths again?" says you.
"Don't go into knitting pattern design" says I.