It's not about buying the office. It's about getting your message to be seen without investing huge resources. No one with a job can afford organizing rallies and laying significant groundwork unless they are already rich, backed by major party or otherwise having most of their time free to do political activity. Ads are cheap way to try to get people to look at your message. It challenges monopoly of the media and gives platform to new players.

I am not sure why you talk about buying votes. Getting people to look at your message is not buying votes. Making crazy promises like free cash for everyone or flats for everyone as human right is.