The advantage of using the FW16E as your audio interface: Sixteen outputs from your Motif XF via FireWire. .
A "normal audio interface" will give you access to two or four outputs from your Motif XF... as you would connect to it via normal signal cables from the Main L/R and Assignable L/R analog outputs.

For a discussion of how the FW16E works, please see this article:

Routing PARTs via FireWire

can you layer sounds from xf with sounds from computer with fw16e?

The FW16 is a fantastic addition. Get it you will not regret it. It's an Audio and Midi interface and simplifies the connection with your computer with just one cable.I have several Virtual Instruments on my IMAC that I layer with Motif voices in a Performance. Remember you have the FW Input for each Performance along with your A/D Input. You can also create a Zoned Master where you control the volume of the Motif Performance with Fader 1; Virtual Instrument 1 on Midi Ch2 with Fader 2; Virtual Instrument 2 on Midi Ch3 with Fader 3, etc. So you have live real time control of the Motif Performance and your computer based voices. Plus if you have Cubase the FW16 is nicely integrated with it as well. It's a wonderful add on to the Motif. If you want to add computer based voices to your arsenal IMHO this is the way to go.

thanks