Why is money so important in elections? How do candidates reach potential voters?

You have two separate, rather large questions here, so
I'll do my best to get to both of them in limited
space. 

Money is important in elections because it buys you
access to the public.  Ads on television are very expensive and you have to advertise
nationwide, so this expense runs into the tens of millions of dollars.  Mainly for this
reason, the candidate with the most money tend to win.  This has been true in the last
three election.

To campaign nationwide requires a very
large staff covering 50 states, all of their offices, copy machines, cell phones and
hotel rooms, and usually a 747 jet to carry that staff around from campaign stop to
campaign stop.  To fire the best staff workers and campaign managers can also run into
the millions of dollars.

As far as how candidates reach
potential voters, there are several methods of varying effectiveness.  Television is the
most convenient, as almost every American has one and watches one.  Radio and newspapers
can also work, but once you get past the basic media, "All politics is retail" as was
once famously said.  This means door to door campaigning, meeting and shaking hands with
thousands of people in key states before the primary and general election, and
participation in town hall meetings and debates. 

To see
how much money we're talking about and where it comes from, visit the link below to The
Center for Responsive Politics (non-partisan).