hmm ok in terms of sparta:
* the most obvious role is that PL provided defense, military and naval support to Sparta in times of war...also provided finances in times of war ie: tribute and contributions were levied on members in times of war
* different allies contributed diff resources to Sparta in war and thus played a role in adding to Sparta's strength.. eg: Corinth was a vital member of the PL and Sparta relied mainly on the Corinthian navy in the 431 B.C Pelop War. It also relied on Corinth's wealth as it had limited funds
* Sparta was obviously the most influential member and hegemon of the PL (ie: Sparta's assembly made up one half of the decision making process) but different states could play a role in influencing Sparta's policy. eg: Corinth was a very important member of the PL due to its wealth, navy and strategic position.. it was partly due to the influence and persistence of Corinth that Sparta committed herself to war with Athens in 431 B.C
* the PL had no specific enemy, it was aimed at stability within the Peloponnese..which obviously held benefits for Sparta (im half guessing here, but maybe this was particularly important for Sparta, as Sparta was prone to helot revolts)
* it was a more stable alliance than the Delian League as it recognised the autonomy of its members.. this gave Sparta an upper hand in the Pelop Wars against Athens.. Athens lost strength and was finally defeated in the war due to the revolts and loss of support of many of its allies
* due to this nature of the PL, Sparta became seen as the 'Liberator' of states in the war, while Athens was increasingly seen as a 'tyranny'
* because the PL supported oligarchies and opposed democracies, it played a role in promoting Sparta's interests. Sparta was in favour of oligarchies due its large, enslaved helot population that needed to be controlled.
lol umm.. i wish i could give u some more info, its just that we never studied the PL in much detail this year.. hope u can find something useful in what ive written
good luck with ur speech!