Purpose limitation means you collect data for specific, explicit, legitimate purposes, and you do not quietly repurpose it later just because it is convenient.
Personal data must be collected for specified, explicit, and legitimate purposes and not further processed in ways that are incompatible with those purposes.
This is where governance projects often go wrong: a dataset collected “to run a process” becomes a dataset used “to optimise, police, market, or influence” - legislation.gov.uk 