Answer:
Consider Athens before public officials were paid. The only people who would be willing to perform public service were the those who could afford to be away from their farms for many days, and sometimes weeks at a time. Ancient Greece (like most pre-industrial societies) was intensely agrarian, and the vast majority of farmers had few or no slaves. Therefore, public offices would consist overwhelmingly of the wealthiest Athenians. Instituting payment would allow a citizen to serve on the Boule or as a Magistrate without having to worry about how he would feed his family, and thus opened those positions to all Athenian citizens regardless of wealth (in theory).
Another of Pericles' reforms was to institute pay for those serving on juries (Note that juries in Athens were very large, often several hundred people). Aside from the aforementioned benefits, paying juries also served as something of a social safety-net. Those who were elderly or disabled, and thus incapable of working on the farm, could sign up their names, and then each morning they might be selected to serve on the jury for one of that day's trials. Juries were selected in this manner to counteract bribery--it would be very difficult to bribe such a large number of people on such short notice.
Explanation:
