Why go to the hassle of towing the float around just plonk a little offshore from your entry point and retrieve it when you come in. No point in towing the flag all around with you as you dive.
Thanks for the historical perspective Sam!
As others have mentioned, the idea is to comply with local laws which requires diving within (I think) 100 yards of a dive flag.
If you are going to leave a dive flag unattended, it may not need to be easily towed. On the other hand, it may not be there when you return and you better leave a good trail of bread crumbs - unless you are super good at navigation. No surfacing to take a peek. LOL