The short answer: jobs.
London predicts that adding weekend tube service from 12:30 a.m. to 5:30 a.m. on the Central and Victoria lines will create 2,000 jobs and pump over $100 million into the local economy each year.
During "Night Tube" hours, trains now run every 10 minutes. In parallel, city agency Transport for London has added eight new late-night bus routes and plans to add overnight service on three additional tube lines this fall.
The London Underground, which started operating in 1863, is the world's oldest subway system.