The power to section is draconian. It alienates the person detained and destroys their trust in the system which is supposed to help them. They lose all rights except the right to appeal and know that if they abscond the police will bring them back.
If they have entered hospital willingly they may be sectioned for the sole purpose of administering medication. They are forcibly medicated if they refuse. However many of the medications do not cure, they only mask the symptoms, and so they may be condemned to a lifetime of drug-taking.
The threat of sectioning can be used to keep a person "compliant" with their treatment even though they may not agree with their diagnosis. Sectioning can destroy a person's confidence and lead to the "revolving door syndrome" where a person keeps returning to hospital because they become dependent on the system.