It doesn’t do what you probably think it does - act as an invisible ‘grid’ for objects in your model.
Instead, it rounds the lengths of things as you draw them, along whatever direction your mouse directs them.
If instead you want to inference to an endpoint or midpoint, edge, or face, you will throw the inference off as it rounds to whatever unit you have set for snapping.
There’s a thread elsewhere which goes into some length about what it might be used for, and the conclusion was ‘almost nothing that’s actually useful - TURN IT OFF - permanently.’ And change and save your templates so that new models based on them start with it off. (It’s on by default in most if not all of the shipped templates.)