A good reason to do it (IMO, as someone who has done it) that it's a lot faster, easier, and often cheaper than designing a fully custom board.

Not only would designing a fully custom board require a lot of error-prone effort, getting a lot of components in tiny quantities can be prohibitively expensive.

Good engineering includes optimizing for cost (monetary and non-monetary), and I'd argue that for many hobbyist one-off projects that won't be manufactured at scale, dev-boards-on-PCB is good engineering.