Undershirts were once a popular piece of baby clothing. Every parent had a well-stocked supply of undershirts and it was the first article of clothing put on a baby after the diaper. It may be that fabrics used to make baby sleepers were rough, not as soft as the cotton and cotton-polyester blends used today. Undershirts would protect a baby's skin from rougher fabrics that could irritate or chafe a baby's skin and add an extra layer of warmth.
Undershirts are not as popular today as they once were. Baby clothes are softer and well-constructed, and most parents don't feel the need to add that extra layer underneath the sleeper. The t-shirt style of undershirts have especially lost favor with many parents as they creep up under the sleeper and create a bulky ridge that is uncomfortable for your baby to lie on. But for some they are still a staple item.
There are many of styles of undershirts ranging from the short-sleeved t-shirt, long-sleeved t-shirt, onesie style shirts that snap at the crotch or snap up over the belly, and many variations. The onesie style are the most popular.
Onesie style undershirts can be worn alone over a diaper in the warmer months offering just enough protection from the wind while not overheating your baby. They do not have to be pulled over your baby's head. They snap down the front and the bottom part snaps up across the belly, or the front and back meet and snap at the crotch. You lay your baby on top of the opened shirt and snap it closed. Some are made of elasticized cotton that stretches over your baby's head and you snap them closed at the crotch, with no snaps at the shoulder or at the back of the neck. These onesies tend to look very nice under pants without the bulk of body snaps.
Typically made of lightweight 100% cotton, undershirts are meant to allow body moisture to evaporate keeping your baby dry and warm. Onesies are usually made of thicker cotton as they are more often worn on their own. All undershirts are machine washable and dryable and wear well over time.
Undershirts are not a necessary piece of baby clothing, but they can be useful and nice to have. A good collection of onesie style undershirts will most likely get a lot of wear during the summer when you and your baby will appreciate them the most.