Most of the places I run nowadays I know pretty well, very well or like the back of my hand anyway but, either way, I’d always take my mobile.
I’m fortunate enough to have the viewranger app on my phone loaded with all of the uk ordinance survey mapping coverage which, providing the phone is charged, works on gps so it isn’t mobile signal dependent. And having access to you’re exact position marked on a mini OS map of wherever you are is an extremely useful thing to have on tap. It can tell me my grid reference, with the option to set it to 6, 8 or 10 digits, or alternatively my position in latitude and longitude and other mapping position metrics that I don’t understand at all but make me feel smug for having anyway
My running vest always has a real compass in one of the pockets and my watch, a Garmin fenix 5, also has a return to start function, giving me a breadcrumb trail to follow back if I ever needed to (never needed yet). The Fenix 5 also has a compass. As does the view ranger app and my mobile phone itself.
My phone also has an additional battery capacity storage in its case so a charged battery is always very likely.
I have the ‘find my friends’ app set up with Hester so she can find out where I am (if there is a mobile signal), as too, not quite so handily, can my daughter in Abu Dhabi and my other daughter in Australia
I can also take pictures on my mobile too, probably it’s most useful function and the best reason of all to carry it. That and important looking up wildlife identification and other things and places we happen upon on the internet
Why wouldn’t you take a mobile with access to all that?