NOI not including Mortgage and Property Taxes?

[Eric Lilly]

Hey everyone! 

Sorry if this is a boring or simplistic question, I'm wrestling more with understanding the concept or reason for this. I'm currently reading "What Every Real Estate Investor Needs To Know About Cash Flow" by Frank Gallinelli. Currently reading about NOI. I understand how one acquires this number. However, he doesn't include the mortgage and property tax payments as an expense. What benefits or uses does this have in comparison to calculating straight cash flow? He talks a lot about evaluating the "income stream" and viewing rental properties as such. That makes a lot of sense to me, I quite enjoy that. But what use is the NOI without subtracting the necessary costs of the mortgage taxes?

For example: If the NOI for a property is 12,000 per year. What can I do with or compare that number to as opposed to subtracting say 9,000 for mortgage and taxes and knowing that my cash flow per year is 3,000?

I genuinely want to know, not trying to nitpick. NOI seems like a really important metric, I'm just trying to understand why.

Thank you all!