lcel benefits (#10898)

docs/docs_skeleton/docs/expression_language/index.mdx

@@ -5,7 +5,23 @@ sidebar_class_name: hidden
 # LangChain Expression Language (LCEL)
 
 LangChain Expression Language or LCEL is a declarative way to easily compose chains together.
-Any chain constructed this way will automatically have full sync, async, and streaming support.
+There are several benefits to writing chains in this manner (as opposed to writing normal code):
+
+**Async, Batch, and Streaming Support**
+Any chain constructed this way will automatically have full sync, async, batch, and streaming support.
+This makes it easy to prototype a chain in a Jupyter notebook using the sync interface, and then expose it as an async streaming interface.
+
+**Fallbacks**
+The non-determinism of LLMs makes it important to be able to handle errors gracefully.
+With LCEL you can easily attach fallbacks to any chain.
+
+**Parallelism**
+Since LLM applications involve (sometimes long) API calls, it often becomes important to run things in parallel.
+With LCEL syntax, any components that can be run in parallel automatically are.
+
+**Seamless LangSmith Tracing Integration**
+As your chains get more and more complex, it becomes increasingly important to understand what exactly is happening at every step.
+With LCEL, **all** steps are automatically logged to [LangSmith](smith.langchain.com) for maximal observability and debuggability.
 
 #### [Interface](/docs/expression_language/interface)
 The base interface shared by all LCEL objects