It seems Joyce Dewitt made the choice to wear pantyhose whenever she had to wear an outfit that bore her legs. This included shorts as well as when she was in a scene wearing a night dress. She wore pantyhose to bed.
Another interesting tidbit about it. Apparently she was in conflict over this choice with the show producers. They didn’t want her to wear pantyhose all the time. Dewitt insisted.
There is speculation as to why she chose to wear pantyhose. One was she was self-conscious about her legs. Another was she believed they offered more modesty in scenes so her undies wouldn’t be on display. You never knew what camera angels might catch accidentally.
Still others seem to think she was a product of her time. Women wore pantyhose. Dewitt might have just thought it was more appropriate.
But ultimately it was her decision. This goes against the previous idea that the show producers wanted her to wear pantyhose, which was the case with Daisy Duke on the show The Dukes of Hazard. Dewitt insisted herself. And it led to her getting pantyhose adverts. Being a pantyhose wearing woman was lucrative for her at the time. Its likely she preferred the extra modesty and she just felt a woman should wear pantyhose.