Do I need a complaints procedure?
If you hold or want quality accreditations — ISO 9001, TrustMark, Which? Trusted Trader, most SSIP schemes — then yes, a documented complaints process with evidence of handling is assessed. Even without one, it is the cheapest way to stop the same fault recurring.
Why record root cause separately from resolution?
Because they answer different questions. The resolution is what you did for this customer; the root cause is why it happened, and it is the only field that prevents the next one.
Can I see complaint trends?
Complaints carry a category and link to jobs and customers, so recurring causes surface rather than each one being handled as an isolated incident.
Does this connect to reviews?
They are deliberately separate. Reviews are public-facing feedback you may want to showcase; complaints are an internal quality record. Handling a complaint well is often what earns the review afterwards.