I begin a brew with certain expectations: Color, ABV, IBU, body Mouthfeel, etc. At every point, I want to know how closely I am reaching those expectations, or not. I am not a perfectionist; if I am off a bit, it doesn't bother me because I know that small variations are, well, small. But if I am way off on anything, I want to know, so that I can adjust accordingly. When I plan a brew, I want the beer to be very close to what I planned. The expectation and the anticipation is what I love about this hobby! It is all about how good the particular recipe is going to be down the line, one or two or three months in the future.
That's why I measure.