Libraries give you a way to share functionality between teams and services. I might create a set of useful collection utilities, for example, or perhaps a statistics library that can be reused. Teams can organize themselves around these libraries, and the libraries themselves can be reused. But there are some drawbacks. First, you lose true technology heterogeneity. The library typically has to be in the same language, or at the very least run on the same platform. Second, the ease with which you can scale parts of your system independently from each other is curtailed.
