They're just a product of a lazy/ineffective management style.
The problem is, proper task delegation requires a lot of work:
1. understanding the problem (probably the hardest problem)
2. understanding the organization and resources
3. breaking down the problem
4. assigning tasks appropriately
5. managing time, both of the process, and the delegates of the tasks.
When a manager does this properly, it's not a very 'visible' process, so nobody really knows they've done all this work.
Alternatively, lazy managers can just call everyone into a room and let them figure out everything. It doesn't require any upfront planning or understanding of the problem. And it comes with the bonus of high-visibility.
TL;DR: meetings are the result of lazy management.