Your hotspot can only be accessed by customers with a valid mobile number. Numbers can be used to track people in case of illegal activities in your hotspot or can be used in an SMS advertising campaign.
In some countries the law requires to identify and track users who use public internet channels. Examples are CALEA laws in the US, Directive 2006/24/EC in the European Union, and there can be country specific laws, like the Pisanu Law in Italy. Compliance with Telecommunication Laws is ensured this way.