Things are done to give the police the tools to fight whatever social ill currently obsesses the political class, however clumsily drawn.
They'll arrest protesters at an event and let them go 48 hours later with no intention of ever charging them. Politicians really want indefinite internment of terrorists. Because they're probably right that nobody will abuse it.
No privacy for drivers.
The age of consent of 16 DOESN'T exist to stop 13/14/15 year olds having sex. It is set at 16 to make (Victorian) child prostitution dead easy to convict. Covertly having sex with any age of teenager who is discreet would be easy. Pimping them would get me in real trouble though.
It is assumed that all these powers will never be abused, because there is so much undetected and unprosecuted crime that the police don't have the time or inclination to abuse them. But they are there when they need them. The British police would never be so ungentlemanly, like those nuclear sub crews who protect their launch enablers with bicycle locks. (http://tvtropes.org/pmwiki/pmwiki.php/UsefulNotes/UltimateDe...) When you have a social contract, you don't need enforcement, just the enablement of enforcement against badness.