You don’t “need” a realtor cash or mortgage. How you pay for it has nothing to do with needing or using a realtor.
In NYS you use a lawyer to help with contracts so the realtor is really only about the search. If you find a for sale by owner, a lawyer can easily do a contract for you. If you find a house that was listed by a realtor, that realtor can help you with the basics of the transaction, just know they aren’t working for you, but they are for the transaction and no matter what they are obligated to be truthful.
If you have a realtor, he can help you do price comparisons of other houses sold in the area, which can be valuable to you, but you can get that information on line too. You can make the contract contingent on an appraisal if you want. With a mortgage the lender requires an appraisal. With a cash deal it’s up to you if you do one.