Update docs/user-defined-functions/index.md

Co-authored-by: Thomas Cooper <code@tomcooper.dev>

Author: piotrpdev

docs/user-defined-functions/index.md

@@ -5,7 +5,9 @@ title = 'Simple User Defined Functions'
 > Note: This tutorial is mainly focused on creating a simple [Flink SQL](https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/table/overview/) [User Defined Function (UDF)](https://nightlies.apache.org/flink/flink-docs-release-2.0/docs/dev/table/functions/udfs/). For detailed information on working with [Flink ETL Jobs](https://nightlies.apache.org/flink/flink-docs-release-2.0/docs/learn-flink/etl/) and [Session Clusters](https://nightlies.apache.org/flink/flink-kubernetes-operator-docs-main/docs/custom-resource/overview/#session-cluster-deployments), look at the [Interactive ETL example](../interactive-etl/index.md).
 
 [Flink SQL](https://nightlies.apache.org/flink/flink-docs-release-1.20/docs/dev/table/overview/) is a powerful tool for data exploration, manipulation and inter-connection.
-It allows you to access the power of Flink's distributed stream processing abilities with a familiar interface.
+Flink SQL has many [built-in functions](https://nightlies.apache.org/flink/flink-docs-release-2.0/docs/dev/table/functions/systemfunctions/#system-built-in-functions), that allow you to extract and manipulate data from the many sources that Flink supports. 
+However, sometimes you need to be able to do operations not covered by these built-in functions.
+In that situation Flink gives you the option of creating your own functions.
 In this tutorial we show how to write a simple [User Defined Function (UDF)](https://nightlies.apache.org/flink/flink-docs-release-2.0/docs/dev/table/functions/udfs/) in Java and use it to manipulate data in a Flink SQL query running on a [Flink session cluster](https://nightlies.apache.org/flink/flink-kubernetes-operator-docs-main/docs/custom-resource/overview/#session-cluster-deployments).
 
 The tutorial is based on the StreamsHub [Flink SQL Examples](https://github.com/streamshub/flink-sql-examples) repository and the code can be found under the `tutorials/user-defined-functions` and `tutorials/currency-converter` directories.