Tunnel mode encapsulates traffic on the WLAN and tunnels it to the ZD, avoiding disruptions to delay-sensitive operations like VOIP. Especially with inter-subnet roaming. So even when changing subnets, the devices in the tunneled WLAN wil stay on the network that the ZD is connected to.
I don't know why tunnel mode would be recommended for VoIP or PDA devices.
(though I'll trust what Jelle has to say)
By default, Ruckus APs bridge wifi traffic directly to the local wired ethernet connection.
Tunnel mode is useful when you want your wifi traffic on a VLAN that isn't (or can't be) carried on the ethernet cable where your AP is connected.
This is common in remote locations or other situations where you want a common wifi SSID/VLAN/Subnet even though your wifi traffic is carried over a routed infrastructure.