Convey was created out of the need to build Pidgin and its supporting libraries on multiple Linux distributions, Windows and MacOS. While there are many continuous integration tools out there, all of them still require maintenance and can only be used by the project itself.
This becomes a problem when you have a community of plugin developers who would also like to build against all of its supported operating systems but can't and shouldn't need to put forth the effort and maintenance involved with using other available tools.
This is where Convey's concept of build containers comes into use. Once you have all the dependencies required to build something, like a Pidgin plugin, you are then able to share that container image with everyone using this system and they are then able to use that image to build against. So a project like Pidgin can supply build images containing the most recent releases with all of the build dependencies for multiple platforms and then plugin developers can use that image to test their plugins against.