It holds things steady while you work.

Edit: 5-13-14
That lead photo always bothered me. It was taken with the camera's "macro" setting and if the subject is too distant, results in a distorted fish-eye view. It looks as if the bracket is curved, bent, or a poor fit... but it's actually straight and square with the world....
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The file is SAE and may appear very tiny in some (metric) software.
To convert to Metric, scale up by a factor of 25.4
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This spindle lock could be used in conjunction with some sort of an indexing mechanism.
http://www.thingiverse.com/thing:124909
It will hold the spindle steady so you can perform light duty operations on a stationary workpiece.

While it manages a pretty firm grip, I hesitate to call this a "lock". Using a torque wrench on the spindle with the thing clamped down required around 15 pound-feet to budge the spindle. That's strong but it's not exactly locked.

I think the clamp-bolt (6x1mm x 50mm) could have been tightened to get a few more pounds of torque, but I didn't want to test it to the point of destruction. Maybe another time.
The hold is very solid, and should be adequate for many operations.. layouts, grinding, etc... but I don't know about something like milling. Needs experimentation.

I used 0.25mm layers, "solid" fill. "Normal" quality, with a raft. It's near a 2 hour print and uses 45 grams of material.

I looked but found no easy way to lock the spindle on these mini lathes. There's just nothing to hang on to. You can't drill into the hardened spindle or chucks. Most people engage the gears / leadscrew etc but I wanted something dedicated.

One downside to this method is a couple of attachment holes must be drilled and tapped into the headstock casting.
The upside is you don't need big fat bolts or a lot of threads to hold the plastic part. I used a #9 drill, went 20mm deep and tapped for two short 6x1mm bolts. That's plenty.

The holding band could be much wider for more strength (maybe), but I designed it just narrow enough to temporarily park it out of the way, back behind the standard (80mm) chuck.

This way you can use the lathe without completely removing the lock.
This might be handy when working between centers, locking and unlocking, or on some project where you don't want to remove and replace the workpiece just to get the lock off and use the lathe.
I still wouldn't leave this lock on there during normal day-to-day operations. Murphy's law.