Many people take drugs because they like the feeling they experience by doing so. This is a more extreme version of why some people smoke cigarettes or drink alcohol. Most do so because they like it and most don’t become addicts.
Because most drugs are illegal, they are traded on the black market, setting their users and suppliers at odds with the law. Because they are illicit and underground, there is little to no quality control, leading to deaths from adulterated or over-strength supplies.
Their illegality makes them expensive as suppliers risk prosecution and punishment, as well as considerable price gouging. The profits to be had from their sale leads to violent turf wars, as gangs fight for control of the trade. It echoes what happened in the United States during the prohibition era. People in the UK, especially young people, are killed in the street by members of rival gangs fighting for control of a very lucrative business.
Several US states and Canada have joined the growing list of countries that have legalized the recreational use of cannabis. If the UK were to do the same, it would lead to better quality control and enable age checks to be made as the illegal market would dry up if the legal market were allowed to prosper through light regulations and licensing. It would free up much of the logjam on courts and prisons, and end the conflict between recreational users and the police. The Treasury, rather than the criminals, would gain revenue.
The legalization of cannabis would take one widely used recreational drug off the black market. The same could be done with cocaine (8.7% of the population) and MDMA (Ecstasy), (1.9% of the population) both in quite widespread use. Their legalization would free up large numbers of police to deal with more serious crimes in which other people are victims.
Heroin was once available on prescription to registered addicts to consume at home, and it was seen as a problem that it could circulate to others. This could be resolved by setting up clinics manned by medical personnel, in which hard drugs such as heroin could be obtained for consumption on the premises, after medical inspection and advice. This would treat addiction as a medical, rather than a criminal, problem, and address it by medical personnel instead of with law enforcement officers. It would bring quality control and safety to the fore, and remove the current illicit drugs trade that underlies so much crime.
It could be argued that legalization would lead to increased use, just as the ending of prohibition led to increased alcohol consumption. US voters went for repeal because the alternative was Al Capone and his ilk. The UK is in an Al Capone situation with illegal drugs, and could similarly end it by repealing the prohibition of them.