Publishers have a lot of web content
HTML, images, Flash, movies
Speed is important part of user experience
Bandwidth is expensive
Use what you need, but avoid unnecessary extra
Personalization differentiates
Show timely data (stock quotes, news stories)
Get accurate advertising statistics
Protect sensitive info (e-mail, account balances)

Not covered:
Proxy deployment is an interesting subject and deserves an entire lecture by itself
Configuring proxy cache servers (i.e. Squid)
Configuring browsers to use proxy caches
Transparent/interception proxy caching
Intercache protocols (ICP, HTCP)