Because you should secure data on laptop's disk and in memory. You also should protect your device from unauthorized installation of programs e.g. keyloggers, program for botnets and mining virtual currencies. Don't forget that you are the product for a lot of companies nowadays.
When you see unlocked device of your colleague or friend open browser on it and open this webpage. The hints increase each visit of this webpage.
We would like to teach people to lock their computers while they are away and improve security.