diff --git a/README.md b/README.md
index 61335a1..7366d11 100644
--- a/README.md
+++ b/README.md
@@ -4,3 +4,7 @@ Internet2 managed CentOS 7 base image
 ## status
 
 ![Build Status](http://jenkins.internet2.jimvanfleet.com/buildStatus/icon?job=docker/base/master)](http://jenkins.internet2.jimvanfleet.com/job/docker/job/base/job/master/)
+
+## purpose
+
+This container allows I2/TIER to insulate itself against upstream changes from CentOS managed images that may be unwanted.  It can also be used to trigger global changes (e.g. a new InCommon.crt) when appropriate.
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