A sleeping bag liner can save you time, money and the sometimes horrible job of washing your sleeping bag. Just smell your clothes next time you come back from a trip and that should give you some idea of how valuable a liner could be.
Bag liners are made in all shapes and it is no problem to find one to fit your sleeping bag. Liners can be found in many different materials but mostly in silk for light weight and moisture wicking properties or cotton which is inexpensive and durable.

Cocoon Cotton Bag Liner/Travel Sheet

Often in the warmer months a liner can be used as a sleeping bag just by itself. A hot night in the south can sometime not get below 75 degrees and its tough to sleep in a standard bag. A liner will make you fill wrapped and protected without over heating you.
Most bag liners add 5 to 8 degees of warmth to a sleeping bag. Sometimes this can keep you from having to bring your winter bag and cut down on your backpack bulk.

REI Lightweight Fleece Bag Liner – Mummy

The sleeping bag liner’s greatest asset is coming home and just throwing it in the wash with those stinky clothes we talked about earlier and not having to worry about what crud you left hiding in your sleeping bag. This alone should have you getting one before your next camping trip.