There are a few reasons why cops might plant evidence to frame someone. For example, if the cop believes that the person they are framing is a threat to society, or if the cop doesn’t have enough evidence to convict them on their own, planting evidence may be one way to convince the jury that this person needs to be locked up. Alternatively, if the cop knows that the person they are framing is innocent but feels like they need to put them in prison anyway in order to “set an example”, planting evidence may be their way of doing just that.