The subsidies are to poor people. The poor people get medical care anyway, at ERs, where hospitals cannot refuse them. Or they forgo the care until mundane problems become chronic or acute problems (and they end up in the ER). ERs happen to be the most expensive means of providing heath care. So, to answer the first part of your question, the subsidies exist so that people who cannot afford heath insurance via the exchanges can still have health insurance. And the reasoning behind it is to reduce health care costs that emerge from lack of access to care.
I know less about the age rating bands. Off the top of my head, it's to smooth costs over a lifetime. So it's a wealth transfer from your young self to your old self, if you have typical health care costs over your lifetime: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1361028/
I know less about the age rating bands. Off the top of my head, it's to smooth costs over a lifetime. So it's a wealth transfer from your young self to your old self, if you have typical health care costs over your lifetime: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1361028/
edit: http://www.rwjf.org/content/dam/farm/reports/issue_briefs/20... for more on age banding.