Why does the writer mention that Lakshmi's shoes "were mended at the toe"?

Mr Kapoor had run the stationery shop outside the bus stand for twenty-two years, and he could count change faster than any machine. So when the girl in the blue uniform came back ten minutes after buying her notebooks, he assumed she wanted to return something.

"You gave me too much," she said, and put a fifty-rupee note on the counter. "I gave you two hundred. You gave me change for five hundred."

Mr Kapoor looked at the note. He looked at his till. It was true. Twenty-two years, and he had made a mistake, and this child, whose shoes were mended at the toe, had walked all the way back to give him money he would never have missed.

"What's your name?" he asked.

"Lakshmi."

He pushed the note back across the counter. She pushed it back to him. They did this twice more, and then Mr Kapoor laughed, took the note and put it in his till.

The next Monday, when Lakshmi came in for a new pen, she found that it had already been paid for. A card was taped to the counter beside the pens. In Mr Kapoor's careful handwriting, it said: Lakshmi, one pen a month, for as long as this shop stands.