Functional cookies remember things that make the site nicer to use: a chosen language, a dismissed notice, a preferred layout, a chat conversation. They are genuinely useful and they are usually not strictly necessary, which puts them on the wrong side of the only line that matters for timing. They wait for consent, and the difference between functional and necessary is therefore the difference between a cookie set on arrival and one set after a click.
What belongs here
Preferences the visitor expressed and would like remembered, interface state, and third party widgets the visitor interacts with such as chat. The unifying idea is that the site works without them and is more pleasant with them. If removing the cookie breaks the thing the visitor asked for, it was necessary rather than functional; if removing it merely makes them repeat a small choice, it is functional.
The chat widget, which is the usual argument
Live chat is the case that generates the most debate, because the visitor may genuinely want it. The distinction that resolves it is when the storage happens: a widget that stores something the moment the page loads is functional at best and needs consent; a widget that loads and stores only when the visitor clicks to open it is arguably delivering a service they just requested. Moving the widget to load on click is usually both the cleaner argument and the faster page.
Why the line matters in practice
Everything in strictly necessary is set before any banner appears. So misfiling a functional cookie as necessary means storing without consent on every single visit, which is the easiest kind of finding to demonstrate and the hardest to explain away. The reverse error, filing a necessary cookie as functional, breaks the site for visitors who refuse, which you will hear about quickly.
Writing it in the notice
Functional is the category where a specific description is easiest and most valuable, because the entries are concrete: remembers the language you chose, remembers that you closed this notice, keeps your chat conversation open. A visitor reading that list can make a real decision in a second. Compare it to the generic description most consent tools ship, which manages to make remembering a language preference sound like surveillance.
Questions people ask about functional cookies
Do functional cookies need consent?
In the EU, yes. Useful is not the test; strictly necessary for the service the visitor asked for is.
Is a dark mode preference functional or necessary?
Functional. The site works in either mode, so the storage is a convenience rather than a requirement.
What if a visitor refuses functional cookies?
They repeat small choices on each visit. That is the correct outcome and it is worth checking that the site degrades that way rather than breaking.
Can I set a functional cookie after the visitor uses the feature?
If the feature is what they asked for, that is a much stronger position than setting it on page load. Loading widgets on interaction rather than on arrival is good practice for this reason and for speed.