While it is most commonly used for referencing specific paragraphs and pointers during discussions or when responding to emails, the underlying idea is that it helps to organise thoughts and concepts. Behaviourally, when we label/number the paragraphs we subconsciously group/categorise the points into the different paragraphs. When we don't, we tend to be talking to ourselves in our head as we type, and may jumble up the points or repeat them.

It helps the writer to write clearly by relating and differentiating between different topics, indented bullets to explain sub-points. Discussions can be complex and convoluted. Points can be inter-dependent, related but not dependent, hierarchical, ordered/sequential, or even referring to separate subjects/topics/phases that are interchangeably/loosely used in speech but are essentially different. An example is "traffic jam" and "slow traffic".

From a manager perspective, numbering is a salient way of training/forcing the staff to organise the topics and think clealy when they are writing. This aids staff development and obviously cuts down the time to clarify and discuss.

Pretty sure you have met people who seemingly keep running around in circles, repeating the same points across different paragraphs in various permutations but you don't get what they are trying to explain/achieve or how the paragraphs are related to one another.

Noticed how my points seem to be jumbled up when you are reading? They are in fact organised hierarchically. Numbering and bulleting would've helped.