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).
Money is of vital importance in the election process. It is the manner in which people can be hired to work for the campaign, engage in direct voter initiatives, and help establish more branch offices in more areas. In terms of spreading the word of a candidate, the more money a candidate has, the more airtime can be purchased in more media markets. At the same time, more advertising space can be bought and this helps to continue to keep the candidate in the public's eye. The presence of money helps to bring out a great deal exposure or publicity to the candidate, allowing them a greater chance of success in running and being victorious.
Money is important in modern campaigns because it allows candidates to identify and to reach potential voters -- both of these are very important.
Campaigns are much more adept these days than they used to be at mining data about people to figure out who they should target. They also can figure out what "pitch" to make to various kinds of people. Doing this kind of research takes money.
Once they have identified who to target and how, the candidates need money to help them do the actual contacts. This can be via mail, social networking, phone calls, or ads. All of these cost money.
Candidates reach potential voters via television advertisements which cost lots of money, hence the reason money is so important in elections.