The old fashioned way

In the spring, when all the grape vine pruning has been done, the growers will fasten the single remaining cane of each vine to the guide wires. They normally use metal fasteners that are applied with an electric stapler-like tool. But, here and there, you see some string.

Whatever was tied with this piece of string is gone now.

I'm thinking that the string is used during the growing season to tie up a wayward stem that didn't get fastened in the early spring. It's just a guess.