Della buys Jim a watch fob because his watch is his most prized possession.
Della and Jim Young do not have much money. Despite this, Della really wants to buy Jim a good Christmas present. She is even willing to sell her hair to get him a nice gift.
This is ironic because we learn Della and Jim both highly prize her hair.
Now, there were two possessions of the James Dillingham Youngs in which they both took a mighty pride. One was Jim's gold watch that had been his father's and his grandfather's. The other was Della's hair.
Della is still willing to sell her hair so she can buy Jim a really nice gift to show him how much she loves him. She searches high and low until she finds it, and then doesn’t think twice before buying it.
It was a platinum fob chain simple and chaste in design, properly proclaiming its value by substance alone and not by meretricious ornamentation — as all good things should do. It was even worthy of The Watch.
The fact that Della sold her hair becomes even more ironic when we learn Jim sold his watch in order to buy her a nice hair set. Della sold her hair to buy him a fob for a watch he no longer has, and he sold his watch to buy combs for hair she no longer has.
One thing is for sure, hair grows back. Della knows her hair will eventually be beautiful again, so she did not mind selling it too much. Her hair was her most prized possession. Once it grows back, she will be able to enjoy the combs. Unfortunately, the watch fob is kind of useless unless Jem gets his watch back.