Due to various security concerns, it is important to be able to build reproducible images. 
For example, using command such as (providing we use the same recipes, yocto distro, meta-layers etc):

$ bitbake core-image-minimal

should produce an identical image regardless of the build directory, host OS, time/date etc.

Currently, the images will differ due various factors, so some analysis is needed in order to determine what causes the differences, which differences can be ignored, which differences can be eliminated etc.