Fundraising is the backbone of political campaigns. Without the money coming in, candidates are limited to what they can afford and not everyone has pockets like Michael Bloomberg.
Ironically, political parties themselves are private organizations, separate from the candidates they are affiliated with. The money they use for conventions, elections, and their overhead, or housekeeping purposes, are subsidized by the taxpayers. This creates problems with corruption and campaign finance laws, especially during perpetual election cycles.