It's a measure of how alive/acive your starter is. If you know what you are doing, you can indeed just wing it and judge things by feel, touch, and smell, etc. But you do need to know what you are doing to be able to do that.

Weighing things out indeed helps and I do that too because it makes things easier. But it's worth noting that people have been baking bread for millennia without access to any hyper precise measuring devices or flour milling and sieving techniques.