It sounds like you build estimation models by looking at the data you have and combining it together to try and figure out your goal.

The disadvantage with that approach you often end up missing factors (because you don't have the data to hand) and end up with a suboptimal model.

In the same way that a lot of startups end up analyzing user behaviour by page analytics rather than user analytics simply because Google gives them page analytics.

It's a good idea to know how to do both top-down and bottom-up estimation models, as best practice is to make estimations using several different models and compare the results.