Clean underwear

Yes....there's still bacteria and icky smells in your undies that have accumilated in them from the previous day.
 
Is there a need to put clean underwear on every morning, if you shower?!

Yes, your body is covered with germs and your pores secrete oil all day, everyday. This mixture gets absorbed by your underwear and no sense in building up layers of it!
 
Cod--
Thanks for stimulating our brains with this deep question.
I like my undies clean and fresh every day.
If you are going to wear the same underwear two days in a row...at least turn them inside-out.
 
I think the only time you can wear underwear two days in a row is if somebody picked you up at a bar and took you home for sex. Even then, you might just want to freeball in the morning. Stuff your shorts in your pocket.