Every single person with kids either wants this, or just doesn't know they want it. That is not a small market. Do you want your kid to have access to your email, saved credentials and saved payment cards? No.
- factory reset the device
- delete all your emails
- dismiss a (very important notification/message)
- spend $$$k on micro transactions
- change a random setting you never knew existed and don't realise why things suddenly stopped working
(I'm thinking here of young kids below the "understand the technology/responsibility well" age, possibly not able to read yet)
Otherwise, they can do things like delete incoming chat messages you haven't yet read, delete any/all notes/photos/ etc, and a hundred similar things — and in many cases, it's easy to do accidentally.