Many barbecues have thermometers in their lids so that you can monitor the temperature without lifting the lid and letting the heat escape. However, the temperature inside your barbecue can vary tremendously from the grate to the top.
If you're cooking low and slow, the termperature where your meat is sitting is very important and can be very different from the temperature that the thermometer on the lid is registering. A remote digital thermometer that sits outside the barbecue with a probe inside is an excellent way to measure the interior temperature without lifting the lid. Monitoring the temperature of the meat is different than the ambient cooking temperature of the barbecue, so you don't want to insert the probe into the meat.
Here's a clever trick:
Take a potato that is roughly the thickness of the meat you are cooking. Slice off a side so that it sits flat. Insert the thermometer probe through the potato so that it is even with the mid point of the meat. Set the potato in the barbecue so that the probe is next to the meat but not touching it. Now you'll be registering the ambient temperature that the meat is cooking at.