Why is Parallel computing important?
Herb Sutter has a fantastic article called on his blog that's 100% relevant to your first question. I highly recommend giving it a read!
In a nutshell, parallel computing is important because we no longer live (and we haven't for quite some time) in a computing world where we can just sit around for a year or two and let Moore's law take care of our performance issues.
CPU clock speeds aren't getting much faster because we've already shrunk our transistors to such an extent that we're butting against the physical laws of the universe. From here on out (at least until quantum computers become a thing), additional speed will come from additional cores. Using more cores in an efficient and scale-able manner can be very challenging.