In order to protect the health there, underpants may be contaminated with various excreta including urine and feces and various germs that may be parasitic.
Every day, these dirty things on underpants will double and accumulate. The longer the time, the greater the threat to health.
Therefore, it is necessary to change underwear once a day. If the situation is special and you sweat more, it is better to change underwear twice a day.
Even functional underpants are recommended to be changed every day. After all, no underpants will have self-purification ability. It is still reassuring to change them frequently and wash them clean.