diff --git a/README.md b/README.md
index a13d38a..5e24375 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,16 @@ table in real time: it streams BGP UPDATE messages from the RIPE RIS Live
 service, maintains a live routing table in SQLite, and provides HTTP APIs for
 querying routing information.
 
+## Rationale
+
+RIPE RIS Live streams the BGP updates that RIPE's route collectors receive, as
+they happen, from the moment a client connects. It reports changes, not the
+routing table they add up to, so it cannot say which AS an IP address is routed
+to now or which routes a prefix has. RouteWatch keeps that table: it applies the
+feed's updates to live routing tables in its own SQLite database and answers
+from them, so an IP-to-AS lookup or a prefix page shows the routes as the feed
+last reported them.
+
 ## Features
 
 - Real-time streaming of BGP updates from RIPE RIS Live