Introduction
Laymen explanation
Traditionally physical lock has been used for home security. Lock is safe as long as its physically monitored by someone. There are great many incidents when theft happen by breaking lock, breaking wall, breaking window. Keeping a guard is expensive as well as not a fool-proof since he may be away for a while or he may went for food etc.
Digital equipment for example, security camera capable to monitor and share incident with owner helps to prevent theft, catch thieves. There are real incidents where criminal were caught due to CCTV footage.
Technical explanation
Reference
http://www.moneywise.co.uk/cut-your-costs/family-life/get-smart-home-security