You need a Care Plan when you start having to explain to others how best to support your elder. If other family members offer help while you’re elsewhere *yay*, it saves you having to explain what to do. The care plan also expresses the current preferences of the person needing extra care.
I’d like to highlight what I think is one of the most important parts of a ‘care plan’. The bit that attempts to convey a little of the life history, passions, interests and preferences of the person being cared for. That way helpers don’t just see an ‘old person’, they see an individual.