What Problem does BuildStream Resolve ?

[BuildStream <edmund sutcliffe+buildstream codethink com>]

I've been re-reading the documentation and thinking about how to explain 
what problem the Buildstream tooling resolves.
   * it recursively maintains the dependencies between repositories of 
source code
   * it is a system that is constructed to build software using build 
software
     [one could say that it is a Meta-Build system]
   * it provides isolation between builds and attempts to ensure builds 
are reproducible
   * it is capable of creation of filesystems which are capable of being 
consumed for
     instantiation of software.

However,
    * Is it a schedule for the execution of the builds ?
    * It doesn't appear to be possible for it to account for who caused 
the build
      to be initialised and the consumption of resources used in the 
build


For someone coming to buildstream for the first time, the questions 
which arise is why should I change my tooling ? What problems does it 
solve better than anything else currently available.. ?

The questions which sort of arise are for example, are why can't I used 
http://packer.io to provided reproducible isolated construction of 
filesystems ? Why is isolation of builds important ? Why yet another 
dependency management systems.. Shouldn't we just use what the Version 
Control Systems offer us.

Thanks in advance
Edmund