Twin Bed Uses
Twin beds are commonly used in children's rooms. They're great for bunk beds, and make the perfect transition for kids getting their own bed for the first time. Most people see them as a stepping stone to a bigger bed or a space saver.
This is a common reason you'll see a Twin XL size mattress in spaces where adult single sleepers commonly lay their heads. In these situations, the standard twin may come up short on leg room and personal space. Twin XL beds are often used in college dorm rooms, hostels or in apartments where sleeping space is very limited.
Full Bed Uses
The Full bed is definitely more equipped for adult living. That extra 15 inches of space goes a long way for young adults who are living on their own for the first time or people who need more space due to limited square footage.
The Full mattress is a staple of apartment living, where extra space is hard to come by. They've also become a popular option for guest bedrooms due to their versatility and size. They can sleep a couple easily for a few nights, though most would prefer a Queen size mattress in the long-term.
Many parents actually choose to forego the Twin orTwin XL mattress altogether and opt for a Full size mattress instead. This can ease the transition from childhood to the teenage years for kids, and give them something they can potentially use into young adulthood. It all depends on the quality of mattress you ultimately choose.