The powers that the police have in any society are a marker of democracy. Too little power inhibits the investigation of offences and simultaneously, as history has shown, pushes police to use unacceptable policing methods, with the ends used to justify the means.
A balance must be struck, giving the police enough power, but not too much, with effective safeguards for each power. The police bring the resources of the state to bear on suspects, who are not yet proven to have committed any crime.
In the face of that, everyone is vulnerable and needs safeguards to ensure that their rights are protected. Layer on top of this the fact that the majority of people arrested by police are inherently vulnerable (for want of a better word) through mental illness, addiction, learning difficulties, age and so on, all of which impair their ability to assert their rights.
In this context, the regulation of police powers, with legally enforceable statements as to what is and is not permitted, as well as clear delineation of the equally enforceable safeguards in place, is essential in a democracy.