Originally, Lightning rods are for harvesting purposes only and have no ability to prevent lightning from damaging crops/trees. Source is from this Feb 4th tweet by ConcernedApe (the dev).
As of Patch 1.7, this was changed:
Lightning is more likely to strike trees and crops, but lightning rods
now have a very good chance of intercepting lightning strikes (if they
aren't already processing a lightning bolt)
So, Lighting Rods can now draw lightning strikes in assuming they aren't charging from a strike already. In this 1.7 patch beta thread, the developer Concerned Ape notes that rods can intercept from anywhere.