Having more Steem Power does not make your posts earn more, unless of course you upvote your own posts which some folks do and others do not.
A percentage of the money you are granting with your upvotes does come back to you in the form of curation rewards. The more Steem Power you have, the more of these you will earn. Steem Power also accrues interest like a savings account. Currently it amounts to about 1.5% APR I believe.
You do not earn immediately from commenting, but it is a great way for people to upvote your comments (earn) and discover you (gain followers to improve future earnings.)
Resteeming also does not benefit you immediately, but curation rewards are partly based on how popular a post becomes, and how early you were in discovering and curating it with your vote. So if you vote on something great and then resteem it, that could lead to more earnings by helping that post become more popular.