Why do farmers register their fields as airports?
Just spoke to the farmer that owns Graham Farms Airport (SN72), a grass strip in a cornfield near the sprawling metropolis of Harris, Kansas. Here it is on the sectional:  

  

And Google Earth:
  

He has a Cessna 182 there. He told me he had it registered as an airport back in the 1970's, like Dave said, in case someone had an engine failure or something, atc could vector them there. He also occasionally allows people from nearby Garnett Municipal Airport (K68) to practice grass strip landings there. 

He got an unexpected benefit from having it registered with the FAA several years back when the power company was planning to run a high tension line right across his landing strip. The power company refused to work with him on it so he called the FAA. Since it is officially an airport federal regulations forced them to run their line around it.  

  

Aviation trumps power company profits!