Wilbur is primarly a wrapper around OpenWRT Buildroot. Building a custom OpenWRT image with a custom kernel is not so difficult if done once, but as long as you need to integrate it into your infrastructure things can start to scatter. Wilbur allows resources like kernel configurations, patches, custom config files to be sticked together and managed as a single, configurable and cloneable entity. Also, Wilbur provides a layer of abstraction over Vagrant and enables repeatable, predictable builds and repeatable, predictable deployments via network boot if your device support it.