although fanservice aside, is it that common to know ur three sizes? i barely know my pants size versus height/overall weight
For guys it typically only comes up for formal dress but is useful when buying new pants/trousers. Knowing your shoulder width, waist, and leg length would be the minimum, cause that affects the fit of your shirt and pants.
For women, I mean there's cup size then band size and the numbers are almost always the band size. That not fitting properly makes bras uncomfortable af. Waist and hip/butt is important in the same way for panties and dresses or pants. Fot example, my wife's waist is about a small or size 1 (used to be size 0), but her hips means she has to wear medium or size 2s.
That being said in most Western countries the clothing numbers are fudged so a 30" waist pants is actually a 32-34". This happens to both male and female clothes.