There are multiple benefits to this - the obvious one is that you can continue to use your primary audio interface's inputs and outputs and use iPad’s at the same time, allowing mics and other external gear to continue feeding into Live along with your iPad. Keep in mind this will introduce a little latency, but unless it's absolutely critical you probably won't notice it.

The other, perhaps less obvious benefit is that you can “preserve” your iOS device as an input for Live even when it’s not plugged in. Say you’re recording from your iPad and have it set up as an input in Live. Normally, when you unplug the iPad, Live will no longer be able to find it and your input is set to ”No Device." If iPad is set up as an input of an aggregate device, however, that aggregate device can remain as an input in Live even if the iPad portion is unavailable. When you plug your iPad back in, it repopulates the aggregate device but there’s no need to reselect it in Live!

One hiccup - whether you set up an aggregate device or not, you will still need to “enable” the iOS device as an audio device each time you plug into your Mac.