NOI is huge when arriving at a value . However if you totally lock in on it , you are forgetting that real estate has 4 Annual Returns !!! 

I echo what Bob "RealMan" Timm and John Juarez said. 

Others have provided helpful responses and Ken Jones has offered to help! 

What a great community. 

For commercial type investment proerties it can be very important. Also important to undestand the numbers that contribute to NOI or your calculation is for not!

I have not used it David Wooley and I was in the business for 20 years with various degrees of investors. Most investors knew a lot more about thier needs then I did and if they didn't they trusted their accountants to know more than both of us.

I prefer to perform a CAP Rate analysis, which includes a gross P&L and is fairly accurate as a determiner for whether to invest and what the maximum offer should be.

The calculation about which you ask are important to appraisers, for the most part. They are not applicable to valuing homes unless the home will be an investment property. I have never used either approach for valuations. 

NOI is important for commercial. Not so with residential and here is why: the inexperienced residential buyer purchasing income property won't understand the application. It is used in appraisals. Comparables are more weighted over NOI in residential- what would a buyer pay for like kind today. Profit and loss is only as good or honest as the person claiming what they spend.  Net Sales less cost of goods sold equals gross margin. Gross Margin minus a-z costs is the net operating profit... and so forth to net profit or loss. That number might be useful to a buyer.

However an easier way to look at residential income property is the standard that a Lender will use. Rental survey minus principle interest taxes insurance HOA and a 25 percent haircut equals net profit. This does not consider depreciation which helps on income taxes...

For investment properties it is important to know these types of numbers.  Few agents get past residential so you many not need to know this.

Formulas shed light on matters at hand to help us make or prove points. Rent, comparable sales and cost to construct plus "math' and other rules of the thumb lead us to the final moment i.e. the calculated risk decision

In commercial real estate, knowing NOI or creating one proforma is essential! 

Nope not me.