Some (many I think) mobile phone plans place limits on monthly hotspot usage. Ie: they'll give you a lot or even "unlimited" on the phone itself, because they know most people won't actually use a lot of data on a phone, but they will cap or soft cap the hotspot. (By soft cap I mean, after some amount of data, they slow it WAY down to basically unsuable. The "unlimited" is often soft capped too but at a higher number.)
