…for both web and mobile... Because not all errors happen on back-end side Because your app may be not the only client of an API Because your users may experience problems, but on API scale it can just be a small bump Because users have a lot of different devices, OSes, versions, brands, browsers, extensions etc Because front side bugs exist Because you can't always find every bug Because you may not test an app that thoroughly, like the most active user would Because you may miss: load testing, latency checks, slow internet testing, low-end device testing