The purposes are the principal objectives for which personal data are collected. These objectives should be clearly identified and personal data should never be processed in a way that is inconsistent with them.
For example:
lead data collected initially to attend a demo session should not be reused to sign them up to your newsletter (except if you did receive their explicit consent to do so) ;
video footages from your CCTV initially filmed to protect the facilities shouldn’t be reused to monitor your employees’ break session.