As said by 2 and 3 money is needed for campaign staff and expenses and media ads.
In a national campaign, the candidate who spends the most on TV spots usually wins. Television advertising is by and large the major expenditure in a national campaign.
In local, small town elections, money is not quite so important, unless the candidate doesn't have enough to print lawn signs. Most local campaigns don't use television. It seems to me that the winner in small towns is the one with the most lawn signs. I'm not sure if the number of signs causes the win, or the number of people willing to display signs indicates a majority support.