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. Or maybe imagine buying a Humvee -- could be handy for some kinds of rural work.

Update: as @SQB pointed out above, Owen originally went for an [R5 unit, which is the low cost budget version of the R2][3]. Perhaps he gravitated to the R5 first out of frugal farmer instincts, but R2D2 (as a used unit) was a tolerable price.


  [1]: http://starwars.wikia.com/wiki/R2-series_astromech_droid
  [2]: http://starwars.wikia.com/wiki/Astromech_droid/Legends
  [3]: http://starwars.wikia.com/wiki/R5-series_astromech_droid