I guess when Charlie Brown went camping last year, he brought an extra tent for his dog Snoopy. That way he could enjoy the comforts of home while away from home.
Usually you'll see him sleeping on top of his doghouse or tent, but in this case he thinks it'd be more comfortable for him to sleep inside the tent. Good thinking, since it does get chilly in the woods at night.

Fan art by me, character by Charles M. Schulz