Maybe think of it this way:
What if you're that coworker who wants to do something nice for someone, and you think other people might want to contribute? Maybe you and your coworkers all know that one person would really love a certain something, and it would make their day, but maybe you (or your coworkers) don't all have the means to spend a lot individually, even if you wanted to. A group gift is a good solution for that. It often turns out to be a pretty inexpensive way to make someone happy, depending on the cost of the gift and the number of contributors.
Also, consider that people quit other people, not jobs. People at work are like your second family in way, since you have to see them at least 8 hours a day every weekday, so why not try to be friendly and cultivate good relationships? A little goes a long way to create a happy work environment. The beauty of gifts is that they aren't necessary – you just do it because you think it'll help someone or that they'll be happier.