The key facts document predates the IPID and some insurers still use both, while others have replaced the key facts with the IPID entirely. Where both exist they serve a similar purpose: giving you a shorter, more readable summary of the policy than the full booklet.
Key facts documents typically cover the main things the policy includes, the main exclusions, any significant limitations on the cover, how to make a claim and how to cancel. They're usually a few pages rather than the twenty or thirty pages of a full policy booklet.
Like the IPID, the key facts document is a summary rather than the policy itself. It gives you a useful overview but the detail that governs how claims are handled is in the full policy wording. If there's a discrepancy between what a summary document says and what the policy booklet says, the policy booklet takes precedence.
The key facts document is most useful at the comparison stage, when you're trying to get a quick sense of what different policies include before you commit to reading the full wording on one or two that look suitable.