I have a close family member who has been sectioned several times, and been admitted voluntarily twice. One on occasion, he was admitted rather later than he should have been. My mother had been pleading with his CPN to get him admitted (she was dealing with him on her own, because my father was working away), but to no avail.
One day, the CPN happened to call round when my father was home for a few days. My brother threatened my father with a knife (because pa wouldn't lend bro £10k to patent his non-existent inventions!) in front of the CPN ad was sectioned within the hour.
Withdrawing someone's liberty, without them having broken any laws, is the biggest sanction we have in our society. It is quite rightly used as a last resort when that person is believed to be a risk to themselves or others.
Capacity is a different matter entirely. If someone doesn't have the capacity to manage their own affairs, someone else can have power of attorney to manage them on their behalf, or the court of protection can appoint someone impartial to do it. Organisations that provide residential care for people with learning disabilities often hold corporate appointeeships so they can manage residents' finances, benefits etc.
People can be sectionable but have capacity, lack capacity but not be sectionable, or lack capacity and be such a risk that they are sectionable.