There are many reasons but just to list few - economy of scale (more scalable, all the expenses will spread over a larger amount of the units), financing you can get a commercial loan with many advantages (assumable, options for supplemental loans ..etc).

I am working full time in San Diego and buying rentals now to supplement my retirement.

My in-laws own rental property in SoCal already (a cash flowing triplex in the San Gabriel Valley) and it would be ideal to follow in that same mould and have properties that are nearby.Our ultimate goal, for now, is to more or less supplement our income, not to replace it.

The advantage is that I can buy another house with cash (and own it - supplementing cash flow drastically).

.,  and then they may supplement that with Excel for billing purposes.  

In a few years time you could end up with a nice passive income stream from 10 leveraged properties.If you are looking for even more involvement beyond that obtaining a real estate sales license would be a good supplemental income stream as well.

There are supplemental origination fees, inspection fees, safety buffer contingencies for the rehab.

I got mine to supplement my income and also to gain access to the MLS.

After I resold the home I received a Supplemental Property Tax bill using the price I paid for the house in June (~$250k) as the new assessed value. 

Although, I have a stock portfolio that has been mildly profitable and I have supplemented my income with stock sales.