I've gotta say that as written the distinction between dative of purpose, "I fight for the king", and dative of benefit, "Every man toils for himself", is a subtle one indeed.
That's exactly mwh's point--these are just categories that grammarians have invented to pigeon-hole uses of the dative, not distinctions that are necessarily inherent in the Greek itself. The categories are useful for drawing attention to the ways in which the dative is used, but they don't really capture the essential Gestalt
of the dative.
The dative is really a very amorphous case--a catch-all for nominal elements of a sentence that have some relationship to the predicate but don't belong in the nominative, genitive or accusative. I would almost go so far as to generalize the use of the dative in a negative way: for free-standing nominal elements in a sentence, i.e., those not governed by a preposition or dependent on another noun (genitive), the dative is the case for any nominal element that is not the subject or direct object or partitive object (again, genitive) of the verb. Thus, the indirect object, the beneficiary, the agent, the instrument all show up in the dative.
Of the categories of datives listed by grammarians, my favorite is the dative of military accompaniment.