The answer to your question can be found at the end of the story. Having sold her hair so that she could buy Jim the present for Christmas that she wanted, Della is shocked to receive Jim's gift for her: a set of haircombs. The irony of this situation is not lost on herself, Jim, and us as the reader:
For there lay The Combs--the set of combs, side and back, that Della had worshiped for long in a Broadway window. Beautiful combs, pure tortoise shelll, with jeweled rims--just the shade to wear in the beautiful vanished hair. they were expensive combs, she knew, and her heart had simply craved an dyearned over them without the least hope of possession. And now, they were hers, but the tresses that should have adorned the coveted adornments were gone.
Thus it is that Jim is particularly sorry that Della has cut her hair because now she will not be able to enjoy the present that he has sacrificed so much to buy for her.
Della's prized possession was her hair. The one thing that she wanted more than anything was those combs for her hair. Jim sold his pocket watch to buy her the combs. Now Jim has combs to give to Della, but she has cut her hair and no longer has a use for the combs.