The most important usecase is when you really want to consume more than 2 gigabytes memory for your process.
The next thing is 32-bit support will slowly decline just like you won't see many 16-bit apps now. This is a slow process though. Currently there's one version of Windows Server 2008 that doesn't have 32-bit support by default.
And finally sometimes you just have to be 64-bit - that's when you build an extension of any kind that is loaded into a 64-bit consumer process. Since 64-bit processes can't load 32-bit code you have to supply a 64-bit extension for them or craft an interop solution to have your code in a separate process - the latter is not always possible and is sometimes very inefficient.
