Apple wants you to buy the Box Set in order to upgrade from Mac OS X Tiger 10.4 to Snow Leopard. Technically, you the Snow Leopard retail package will upgrade intel based Macs running Mac OS X Tiger 10.4, however it is against the terms of the End User License Agreement (EULA).
The Box Set includes additional software that typically comes with a new Mac (and some that doesn't) and it is recommended by Apple to 'upgrade your Mac experience' because it is more like buying a new Mac than just upgrading the operating system.
To hear it straight from Apple, here are the answers from Alan Eyzaguire, director of software product marketing at Apple Europe to this question.
"We just think that Apple customers could really benefit from having those. They're great software."
"it's great when people get the full experience"
"if you have Snow Leopard it's great to have the latest iWork and iLife"
"we wanted to make that [upgrade] as attractive as possible"
"you get it almost for free"
"it's a steal!"
"Technically, yes, it would upgrade a Tiger install, but in the licensing, no." (citation)