iOS turns off the hotspot when no device is connected.
This becomes irritating when I'm using my iPad as a hotspot in a foreign country with a prepaid SIM card: when I put away my iPhone, the iPhone disconnects from the hotspot and the iPad stops broadcasting. I'd then have to turn on my iPad and navigate to the Personal Hotspot setting to be able to tether again.
I can understand it's to save power, but with 25 hours of battery life, there's really no reason I have to fiddle with my iPad each time I need to connect to the hotspot after a period of non-usage.
Exactly, its rather annoying, that is why I chose to buy a cellular iPad air so I can just share my monthly data allotment for $10/mo and have my iPad always connected whether I'm in wifi range or not. it goes everywhere with me.