Use /var for storing webpages.. Why???

[Adam]

In Greg Lehey's book "The Complete FreeBSD" 4th edition, he says, "Do not have a separate /var file system unless you have a good idea how big it should be.  A good example might be a web server, where (contrary to FreeBSD's recommendations) it's a good idea to put the webpages on the /var file system." p.70

Why is it a good idea to put webpages in the /var file system and not the /usr file system?