Just want to start this off by saying if this question comes across as insulting that is definitely not my intent. I'm only asking this for informational purposes. No harm intended.

I've been doing a lot of research on properties for sale in my local area. Maybe looked at over 100 properties. Another investor mentioned to me that I need a realtor. But why? Why do I need to use a realtor? What can they do to benefit my search for investment properties? 

@Craig Matthew   Not everyone needs to a realtor. If you find a good one that invests themselves, they could be a great resource of knowledge, pocket listings, and find value in properties you may have otherwise overlooked.  If you know what you are looking for and you are out there hustling, then you could probably get by just fine without one. There are investors who like to use dual agency (use the selling realtor as their own) because in a way it would incentivize them to choose you if there were competition because then they would get double commission.  You could also leverage that as a way to negotiate a lower price with some agents. 

@Craig Matthew you won't be able to do much with the MLS without a realtor. You will need one to view the properties. A good realtor will be invaluable purchasing on market properties for a few reasons, they can bring deals to your attention as soon as they come on, they probably know of properties that aren't even listed on the MLS. They have market knowledge and relationships with other agents, contractors, insurance people etc. They are there for your best interest and to protect/advise you. As a buyer they cost you nothing, so the price is right... just be respectful of their time

I've closed deals with and without a realtor. I would not recommend going alone on your first few purchases. There are too many variables, and until your business resources are strong enough to withstand a costly error, it's best to bring a professional along for the ride. And I strongly agree with @Daniel Vandenbos , it's important to be respectful of their time and recognize their minimal commission on most investment properties. Best wishes on your journey - sounds like you're asking all the right questions!

If you think you can do the job better than a realtor then i guess not? Using the average Joe Realtor from the big brokerage house may not provide value to your needs but its up to you to identify your needs.  If you want a second opinion on analysis from someone who has experience with the property type, off market deals (pocket listings) and a sphere to reach out to then why not? 

If I find a property off market for my client it provides a WIN WIN WIN. 1. The sellers net with one broker is higher than listing on the active market with a broker. 2. The buyer also get a lower price. 3. I get paid more than being just the buyers broker. For example, rather than a 2.5% CBA, we negotiate 3% and I drive the deal. Use the principles of negotiation to lay this out to prospective sellers of properties of interest. 

Find the right broker who has the right tools. I specialize in Multifamily and have a database of ALL multifamily properties in 5 counties in the State. This is a valuable tool in identifying properties during an exchange or tailored out-bound marketing finding a property size/units/price/condition that you're looking for. Imagine a resource that could narrow down a database of 10000+ properties to 100 and is working for YOU and at the end of the day you don't need to pay for.