I went to business school and dropped out after 3 months. For me, it turned out to be a waste of time and money. But that doesn't mean business school is useless for everyone (which is the popular opinion in Silicon Valley these days). It's been a little over a year since I've dropped out, and I can think of 3 good reasons to attend.
Business School facilitates industry and/or functional career transitions. While it's typically difficult to change both industry and function (ex: energy sector analyst to sports marketing), it's even possible to achieve such a dramatic change by taking some small steps (academic year internships, etc.) over the course of two years. If changing just industry or function, the transition is relatively straightforward.
While it technically wouldn't be impossible to make such transitions without the significant financial and temporal commitment of business school, what these programs offer is considerable de-risking of the transition. It virtually guarantees a soft landing in even the worst case scenario even if things don't work out perfectly.
Yes, there are people who can make these transitions without another academic line item on their CVs. But not everyone has the track record, network, or drive to make it a reality.
For those coming from an excruciating career start in places like banking or consulting, business school can be a 2 year respite that won't leave a blank in the resume. Students who are sponsored by their employers won't even have to fork over the tuition that can be prohibitive to many. Even without this, they may have enough of a cushion built up and have very good financial prospects coming out of school that the financial downside can be mitigated.
Terms like "funemployment" are thrown around casually in Silicon Valley, but not every industry or career trajectory will let people randomly take 12+ months off and come back to the workforce seamlessly. "I was working on some side projects" doesn't (a) have much credibility and (b) isn't even possible for some functions (ex: corp finance) for most careers outside of software or design.
For many industries, not having a blank period in the CV can be worth a lot of money.
The business school alumni network is a value-add that I understand in theory, but have yet to see used in the wild. However, I have been using my undergrad alumni network from time to time, and thus I can see how people without a super-nepotistic (i.e. helpful and supportive) alumni network like that of my alma mater would see a great benefit to being part of an alumni network that seeks to help each other out as a kind of a fraternity.
Further, no matter who we are, our current network of friends and professional acquaintances tends to encompass a fairly narrow range of backgrounds. A business school class of roughly 400 students covers a very wide range of industries and functions. The ability to quickly tap the knowledge and perspective of someone in the trenches in practically any sector can be an asset.
For example, when I needed a sanity check and first order guidance for what to do for year end tax compliance at my startup, I was able to chat with my business school friend with an big-four accounting background to confirm what next steps I needed to take.
This is likely not an exhaustive list of reasons why business school might make sense for someone. But what I learned through my own experience enrolling and dropping out is that it's critical to have something concrete that you want to gain over those 2 years that would be very difficult to obtain otherwise. I only had vague, weakly held ideas that brought me there, and I paid the price for it (literally).