> I couldn't imagine trying to build a complex route on a tiny screen with a crappy phone UI.

I nearly always plan routes on my phone. After all, it's the device I use for navigation attached to the stem of my bike.

> Why in the hell wouldn't you prefer to use a nice monitor with a mouse for a complex task like this?

The task isn't very complex with a decent UI. See for example the open-source app OSMAnd (Android & iPhone). Also, I am rarely at my desk, but I always have my phone on me. There can also be the issue of "Neat, I made a route on a big screen; now, how do I get it on my phone for real-time navigation?" depending on the app you are using.