There are times when an expedition (cold temp) sleeping bag is overkill and a summer bag is not enough. Or if your budget does not allow for a colder temp bag, a bag liner may be in order.
A bag liner may increase the temperature rating of your bag from 5 to 15 degrees (F)
There are several materials used for this purpose.
Some are much more packable than others. and warmth varies.
These liners can also help keep the inside of your bag from getting dirty and grimy. As a bag (especially down) is diffcult to wash but a liner is a snap.
A bag liner can also be used as a primary sleeping bag in warm conditions.
Overbags can also be used as an outer layering system in colder conditions.