What is the purpose of underwear??

This topic came up while I was having a conversation with AlexC last night. Why DO we wear underwear?

Undies do serve a purpose for some:

  "They keep outer garments from being soiled by bodily secretions and discharges, shape the body, and provide support for parts of it."

Some people have other ideas:

• To protect "expensive" garments.
• To eliminate dreaded panty lines.  
• To reveal your sexy side.
• So parents can yell at you to wear them.
• Visible panty lines. We need 'em. We really do.
• Aesthetics maybe?

What do you think?