Answer:
Crafts once fulfilled useful purposes, but now crafts are used mainly for decoration.
Step-by-step explanation:
In the original statement "Crafts once fulfilled useful purposes, now crafts are used mainly for decoration", the statement presents two opinions. The first part of the statement presents what crafts used to be, and the second part gives the present-day condition.
These two contradicting or contrasting statements can be rewritten by using the conjunction "but" for they are two different things though still containing the same idea. And if we use "but" to join the two parts of the sentence, then the comma will come right before the conjunction.
Thus, the correct revision will be
Crafts once fulfilled useful purposes, but now crafts are used mainly for decoration.