I am thinking of getting a Tascam US-20x20 interface. This has a Word Clock connector at the back.
 
Can Sonar take advantage of this? The only timing settings in Sonar that I can find have to do with Midi.
 
I also have an Akai Z8 sampler which also has a Word Clock.
Will my Z8 sound better if I also connect it up over Word Clock?
I have read up on Word Clocks, but it's all theory and I don't understand the practical benefit because 
I have always just connected the Akai Z8 over the analog jacks and I have never noticed any problem in the sound.
 
I also have a bunch of Roland and Yamaha sound modules, and they don't even HAVE a Word Clock. I also have always just connected them direct.
 
I also have a Behringer ADA 8200 with a Word Clock. I bought it, thinking I could use it as a preamp. 

The audio interface will sync to word clock so you'd just select the audio interface as the clock master.

If your using analog connection then word clock is not used. It is when you connect devices digitally that then only one device must be a master, and therefore the connection that is used for this. 
Example my interfaces all have SPDIF, And I also have a Yamaha 01v with SPDIF. 
The yamaha will ask me if I want it to be the master or the slave when I use that connection. 
In most cases I let Sonar and the interface drivers be the master. 
The word clock connection is used if you have a dedicated studio clock. Small studios normally don't. But the big studio need to keep everything in sync so it's almost a given they will use one. 
So if your sampler has a SPDIF or TOS connection and so does your interface, you should use that to aviod double conversion. There are many digital conections these days but SPDIF is most common on interfaces. 

yep - word clock will only work if you are using the digital ins and outs between the pieces of gear.
 
The word clock is basically managing the timing of the digital signals between your digitally connected devices so they are all working in sync with each other.
 
SPDIF or the ADAT optical can both sync to another device through their own data streams - word clock is just another option.
 
Upside - you gain more analog inputs to buy more gear!
 
Downside - another layer of cables and another area to trouble shoot when your digital gear starts sputtering.