Can anyone tell me the purpose of the purple line in this picture?  I've always attached it to the tack of the spinnaker, but don't really know it's purpose.  Also I'm not sure that all boats have this rope.  

If you rig the kite , hoist and drop, the pole on a 200 will still be out by around 150mm. You then push the pole all the way back, attach the line to the kite tack 'tightly', and so next time the pole fully retracts. 

As well as keeping weight marginally further aft, it stops the pole catching on other boats as you bear away behind them ;-)

Practically speaking, it just means everyone isn't sailing around with long jousting poles! Having the pole further in means you can get closer mark roundings and ducks to other boats as well, so I'd say it's an advantage to have pole fully retract.  

There's also a rule that you can't modify whats in the pole and all boats should be able to deploy the SRS by adding a tweaker line, however, on the new boats they don't have any of the gear inside the pole or with the mast.  I've removed all trace of the SRS on my boat, mostly because re rigging after I had to remove the pole once was a faff. The class rule below probably needs deleting. 

If not using the SRS you can set up the tack very simply and effectively. There is no need for a stopper knot and bowline like seen in the picture you posted, simply push a loop through the eye, then a bobble through the loop http://www.burghfieldsailing.org/index.php?module=pagemaster&PAGE_user_op=view_printable&PAGE_id=225&lay_quiet=1" rel="nofollow - like described here for a mainsail . The trick is to pull a lot of tail through with the bobble, so when it pulls tight, the line is pulling the tack straight rather than twisting it. 

Remove the end of your pole feed the tack line through it, then tie a stopper knot. If you're tack line is very thin (3mm) then you may need a bobble here. This knot sits inside the pole, and limits the amount of tack line that can come out, so pulls the pole in when the spinnaker pulls back into the sock. The knot inside the pole needs to be about 20-25cm from the end, you may have to play around with it to get it right. 

Feed the tack down the pole. Depending on when your boat was made you can tie it off in different places. The older boats have an aluminium plate with a loop which works. I've taken this off my boat and it now fixes to the kicker eye. What works really well is having an adjustable splice here. On light wind days you can lengthen the tack a cm or two so that it doesn't foul the end of the pole. 

Then, pay attention to how you tie the head of the kite. You need a stopper knot here, then a bowline which hold the head of the kite 2-3 inches away from the mast. Depending on how raked you sail, and how you like your spinnaker to set you can change how far you have the head sitting off the mast. Further away you have the head from the mast, the more you can 'project' the kite away from the mast, and the further you can float the kite round to windward when aiming for depth in light winds. Tie it too far away from the mast and the kite will start to droop as if not fully hoisted, particularity on reaches, or when windy and sitting out the kite will drop to leeward and will start pulling you sideways more than forward.  

There's a good chance you didn't know you had one because you don't have one. 

If you have a newer boat it's likely it came fitted with a stopper knot inside the pole (like described above). A lot of older boats have this modification too if you bought second hand. 

That's a good bit of info there.  I must admit while I've seen people rigging spinnakers with bobbles, I'd always thought it seems to be asking for the bobble to come undone when not under load, but I guess people wouldn't do it if this was an issue.

Here's a video; sorry it's a bit dark. 

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