the firewall on your router blocks unsolicited incoming calls ans some of the less used outgoing - but the one on your pc blocks unwanted incoming but also asks which progs can do their thing (or at least should do)
so rtr firewall makes it hard for hackers (as does the dhcp server) to get in thus protecting the whole network from nasties - sort of locks and opens doors (once open anyone can get in)
pc fire stops individual progs doing their own thing without your permission. which sounds the same - but gives finer control to "ordinary" users (sort of guest list)
so open port 80 - the router will let any internet browser access web info
the pc however will stop nastyMacProg from using the connection but still let IE access sites
if the rtr blocks port 1234 - nothing gets out - from any part of your network
if your pc stops firefox - I can still use it on my pc