Everything has a purpose - except...

Could it be to fasten the cable back onto itself in a loop to act as strain relief, to provide some cushioning so movement of the charger on a taut cable doesn't bend the cable close to the moulded-on plug or pull it out?

Or is it so you can clip the plug end to the charger end when you are packing up the charger to put in your laptop bag?

I don't think it's that.

It's more likely to be that ... an alternative to the Velcro strap.

I rarely use either as they often put excessive bend in the cable where it exits the PSU, causing the wires to fracture at that point (not so bad for the mains lead as they can often be replaced as they plug in).

Cheers, T i m

No but many do wonder what the solid bit is on many power supplies on the lead. its a feeble attempt to get rid of RF radiation from either the laptop or the psu. In my experience it takes more than a ferrite ring or two to snuff out the crap most things put out these days. Brian

Shirley it's gonna be so when the cable is not in use, you can coil it up and clip the two ends together.

My MacBook Air has such a clip on the cable at the magsafe connector end. I wrap the cable round the charger and clip the end to another part of the cable. Stops it unwinding.

on 22/11/2017, Tim Streater supposed :

I think that has to be the answer, but I never thought to try using it for that.

Exactly. In fact some of the Mac chargers are quite smart, with the velcro and the C clip you can organise them so that spare mains and DC cable is wound up neatly so that if you have a socket close to your laptop site, the "plug to plug" dimension is optimised.