Possibly competitors of the advertiser [who can't afford PPC]? Another thought: An advertiser could use a bot to click on ads for keywords they are targeting.

Possibly competitors of the advertiser [who can't afford PPC]? Another thought: An advertiser could use a bot to click on ads for keywords they are targeting. If they could exhaust their competitors daily budget they could potentially get higher placement for less CPC [later in the day]. I'm thinking of Adwords here.