- low-bandwidth availability, particularly in less-developed regions of the world
- censorship evasion
- security concerns. some users want to access Wikipedia without exposing their machine to the internet
There are probably a few others I'm missing....
Unfortunately, having your own copy of Wikipedia could also be used to enable censorship. For example, a fundamentalist school could have their own version of Wikipedia from which they've purged all articles about evolution, etc. Then they could configure their firewall to block the real Wikipedia.
By and large, for most private individuals, an offline app would allow them to evade censorship. I'd hope that this benefit outweighs the risk of the other's abuse.