It shows that she is an amazing wife, and would do anything for her husband. But when he sold what he had to get HER something, she didn't get mmad or anything.
Della's financial situation constrains her, but her selflessness helps her overcome that constraint. She loves and understands Jim well because she knows that the best Christmas gift for him would be the chain for his prized pocket watch:
It was even worthy of The Watch. As soon as she saw it she knew that it must be Jim's. It was like him. Quietness and value—the description applied to both. Twenty-one dollars they took from her for it, and she hurried home with the 87 cents. With that chain on his watch Jim might be properly anxious about the time in any company.
The strength of Della's love and her willingness to sacrifice for him is clearly demonstrated when she cuts off her hair to buy the chain. Her hair is something she values, and sacrificing it possibly made her undergo an internal conflict:
So now Della's beautiful hair fell about her rippling and shining like a cascade of brown waters. It reached below her knee and made itself almost a garment for her. And then she did it up again nervously and quickly. Once she faltered for a minute and stood still while a tear or two splashed on the worn red carpet.
