well when you moving around code you want to be in normal mode. so lets say you cursor is at the top of the function which would be 0 (with relative numbering) you could simply look down and see how many lines you need to delete.
so for example lets say 5, then you could simply do 5dd or 5yy, without having to count all to see how many lines which can get annoying.
while in insert mode you would simply want to see what line you are one and for collaboration with others. In cases where you might want jump to a line like :15.
There is also a NumberToggle function if you ever need to switch on the go.
If you're operating on more than just a couple lines, then it's probably a bad sign to be doing math. It's usually better to navigate to the end of the selection and set a mark or select the text in visual mode.
while in insert mode you would simply want to see what line you are one and for collaboration with others. In cases where you might want jump to a line like :15.
There is also a NumberToggle function if you ever need to switch on the go.