After the formerly blind man is brought to the Pharisees, John reminds readers that it was the Sabbath when Jesus made the clay and opened his eyes. The dispute therefore concerns not only that a healing occurred but what Jesus did to accomplish it.
The physical act becomes legally significant.
The Torah commands cessation from work on the Sabbath but does not provide an exhaustive biblical list covering every possible activity. Jewish interpreters necessarily applied the command to practical cases. Later rabbinic sources discuss kneading and healing among detailed Sabbath categories, but their final formulations should not simply be projected backward unchanged into the first century.
John's explicit mention of making clay suggests that Jesus' action itself was viewed as work by his critics.
The background explains why the clay is not an incidental medical detail. It becomes part of the public argument about whether Jesus can be from God while acting this way on the Sabbath.
John then lets the extraordinary fact of opened eyes challenge the critics' categories: the sign remains before them even as they dispute its legality.