Booby trap in a "meth" house

Why would you want to set a trap for people walking up the stairs?

Downstairs I get. Cops, rivals, bailiffs, evangelicals, mom or basically anyone trying to harsh your shit when you’re gettin’ your buzz on. You want to get them when they’re coming down the stairs. When they’re going back up the stairs (possibly with you leading with your hands behind your back in a zip tie) it’s too damn late.