Add aims to documentation

README.md

@@ -14,7 +14,7 @@ peer reviewed evaluation tools and provides a legacy for diagnostics and
 observations. It offers flexible evaluation code that users can easily adapt
 according to their needs. It is build on a modern software stack using Python3
 and Met+ in a conda environment to support portability. Clear documentation,
-working practices, automatic testing,  and open access ensure developers can
+working practices, automatic testing, and open access ensure developers can
 easily use and contribute to this system. 
 
 Please read [the documentation](https://metoffice.github.io/CSET) to learn more

docs/source/background/why-cset.rst

@@ -15,8 +15,20 @@ CSET is a centralised place for custom diagnostics to live, with well defined
 :doc:`/contributing/index` to ensure that they stay maintained. By contributing
 diagnostics to CSET you ensure that they outlive the paper they were written
 for, and benefit the entire modelling community. Furthermore, any operators that
-the community finds especially useful will be contributed back to
-METplus_, so they can serve an
-even wider community.
+the community finds especially useful will be contributed back to METplus_, so
+they can serve an even wider community.
+
+It aims to align parametrisation, diagnostic development and evaluation research
+linked to Met Office Research Atmosphere Land suites (RAL) across the Met Office
+and UM partnership. Apart from verification capabilities it focusses on
+providing and continuously developing a centralised source of tools to aid
+process-oriented evaluation for UM and LFRic systems supporting deterministic
+models and ensembles. It supports community development of well documented and
+peer reviewed evaluation tools and provides a legacy for diagnostics and
+observations. It offers flexible evaluation code that users can easily adapt
+according to their needs. It is build on a modern software stack using Python3
+and Met+ in a conda environment to support portability. Clear documentation,
+working practices, automatic testing, and open access ensure developers can
+easily use and contribute to this system.
 
 .. _METplus: https://dtcenter.org/community-code/metplus