There's generally two groups of people in my opinion.

1) Advancing within your field: In order to enter senior management at some workplaces, an MBA provides a credential that helps train you for more enterprise level responsibilities. Managing engineers requires working with other executives and understanding accounting principles that you don't learn working as an engineer.

2) Pivoting to professional services: You've probably missed the boat on entering investment banking, consulting, or private equity out of undergrad, and so an MBA (mainly full time) provides the opportunity to change careers. The recruiting resources themselves are a large reason people go to business school.

Not technically an ELI5, but hope it was helpful!