What benefit of interface bundle on BIG-IP high platform?

When you assign interface to a vlan, it only becomes a member of that vlan and has access to whatever info passes through the vlan, just like on a regular vlan network. However, when you assign interfaces to a trunk, you are bundling interfaces together to form a single link, hence, increasing the bandwidth available for any traffic that would be sent down that link.

Additionally, if any member interface of a trunk goes down, the trunk is still active as long as it still has active interface(s) to handle traffic. The strength of a trunk is the sum of the strength of the individual interfaces that make it up.