If you've been drinking the type or particular tea for sometime, most of the time you get a pretty good idea of how much tea/grams to put eg. half or 1/4 of your daily teapot.
Basically the idea of measuring is to start out to find the best ratio for a tea, eg. aged raw slightly more in proportion to young raw etc. Measuring is also useful if one is comparing between 2 or more tea to get a standard so to speak.
This.
I'm confident I could get a good amount of leaves from a familiar sheng in my familiar yixing, just from a little practice. Even if it's off a gram in either direction, big deal - that bit's kind of fun, and I do it on purpose anyway, just to mess around with the flavor and strength.
But, starting out, or with a new tea, or pot - it makes sense to me to measure and go from there.
But we're circling around