Alcohol in low to moderate quantities helps people in social situations. It makes people more, how to say, myopic. A little drink and you don’t think as much about long term consequences. This makes it easier for people to talk about some things, or take risks like asking your date if they want to make out. Excessive drink will fuck you up, though, especially over a long time period.
Drugs are many and varied. I don’t really use any so I can’t speak to them, but generally they make people feel great. There’s a huge cost to pay for some of them, but people chose that for various reasons.