Partially for propaganda reasons. They were aware that she claimed, and many among the French belived her to be a messenger from God himself. Morale among forces of the Dauphine had gone up, and Jeanne d'Arc had led them to several victories, confirming in the eyes of many French her claims to be on a mission from God.

To merely execute her would, in the minds of many French, be to kill a representative of God.

However, claiming that she was not a messenger from God, not on a mission from God and that the victories she had led the French troops to were not the result of divine favour, but dark and heretical witchcraft could perhaps turn the morale boost of the Dauphine's forces into the opposite.

In the end, it did not matter, the Dauphine won the war and the British were (apart from Calais) forced from France, but the decision to hold a trial and expose Jeanne's suposed powers as witchcraft rather than divine blessing made sense to the English at the time.

The British didn't because Britain wouldn't exist for a few more centuries, not until the Act of Union in 1707.

Just a bug bear of mine.