I agree it seems strange.  R2 droids are [specifically engineered to work on starfighters][1]

> this rounded, waist-high droid was made specifically to fit in military starfighter astromech slots

However, it probably retains the general purpose of an [astromech droid][2]:

> a type of droid that served as an automated mechanic, performing a variety of repair duties 

which means it would be handy to have around in a semi industrialized agricultural environment.  We also see that R2 series is very high quality.

So this is a bit like buying the highest quality Swiss Army Knife -- sure it's engineered for military use, but it is handy in many other situations.

  [1]: http://starwars.wikia.com/wiki/R2-series_astromech_droid
  [2]: http://starwars.wikia.com/wiki/Astromech_droid/Legends