Ironing Undergarments
Believe much of that was done before tumble dryers became common place in many homes.
Undergarments until the invention of modern man made fabrics were usually made from cotton, linen, wool or silk. All of which feel and look better after line drying if given a touch with an iron. Also most of these garments were of woven rather than knitted fabrics (except obviously wool and perhaps silk). Woven fabrics wrinkle and crease more during laundering than knits.
Both knits and woven fabrics however will come out of the dryer decent enough for most households. This explains why ironers and ironing declined rapidly in the years after 1960's. The long list of bed and table linen along with other garments and household textiles that were once ironed as a matter of routine after laundering, now were merely tumbled dried.
About a year ago purchased several sets of NOS cotton pajamas made in the 1950's. While one *could* wash and tumble dry, they just don't feel and look the same as when laundered and ironed.