(The other coauthor here.)
Teddyh's answer describes some of the technical aspects, through I would like to add the security scenarios that Mandos works to address. Any security measure is in one way or another designed with known threats, assets and costs/outcomes.
If one operates a bunch of servers with FDE in a server room, getting there every time there is a need to reboot is a significant problem. To mention a few causes, redundant nodes going up and down in the middle of the night, updates to the operating system and kernel, and misbehaving hardware. At the same time, those servers are likely to hold a lot of sensitive data to companies or persons, especially email, which puts the administrator at conflict between using full disk encryption or not using it. In my experience, unless there are regulations that dictate otherwise, servers are not encrypted because of the hassle and downsides of manual or needing to attend reboots in person. This was the initial case as to why Mandos was created many years ago. If the server hall loses both primary and backup power, there is a real risk that the administrator does need to travel there to bring the machines back up. That would be one of the major trade offs, through I would still recommend administrators to do that, compared to the risk of an unencrypted disk getting lost, stolen, or cases where someone comes in and takes all the servers.
There are naturally other scenarios that one can use Mandos for, but like any tool it's good to know whats it is designed for. It is not intended to replace setups where one is already using FDE where one types in the passwords manually at the terminal and is happy with it. If one does not need the unattended aspect but want to remotely reboot the server, there are things like Dropbear or IPMI/remote KVMs, in which case the security will rely on those components' security. In my experience, IPMI's security should not be exposed to the internet which means one first needs a security entry point to the local network. Dropbear uses ssh which mean one should use client certificates and verify the signature before use. Depending on the use case and what risks one wants to take there are benefits and drawbacks, but the key point I do want to come back to is that people really should use full disk encryption and Mandos alleviates the primary reason people don't use FDE.