Making sure temperatures are correct for our little shelled friends is an absolutely fundamental part of tortoise keeping. 
Temperatures dropping too low or reaching dangerously high levels can have devastating consequences. Precise control is very easy with a digital thermometer on hand. 
These come with a probe to measure the temperature inside the enclosure while the unit with clear display can be fixed to the front of the enclosure to easily check that everything is within good temperature range.
**Please check the batteries are firmly in place when they arrive with you, just in case they have come loose in transit.