And he (the devil) has no authority on them, except that (through his instigations to human beings) We (come to) know (or distinguish) him who believes in the hereafter from him who is in doubt about it, and your Fosterer is a Custodian over everything. (R 2)
Believers do not follow the devil whereas infidels fall into his trap. The devil thus helps in distinguishing believers from the infidels and this appears to be one of the purposes of creating him.