Improve index intro

Signed-off-by: Philippe Ombredanne <pombredanne@aboutcode.org>

Author: pombredanne

docs/source/index.rst

@@ -7,6 +7,28 @@ package catalog. It works without a network connection after installation.
 A valid PURL string can still name a package that is not known. PurlValidator
 adds this package identity check for SBOM, VEX, SCA, and compliance workflows.
 
+Why?
+-----
+
+Package-URL, or PURL, is the de-facto standard for identifying software
+packages, used by open source SCA tools, SBOM and VEX specs, and vulnerability
+databases. But using a standard syntax does not prevent errors: A recent
+study on the quality of software bill of materials (SBoM) revealed that for too
+often PURLs in SBOMs are still inconsistent, fake, incorrect, or misleading.
+This is a major impairment to any application of SBOMs, and industry-wide
+cybersecurity and application security.
+
+The PurlValidator project is a public service, based on PurlDB, to validate all
+the PURLs. An extension of the purl2all project, PurlValidator validates the
+PURL syntax against any known PURLs by exposing PurlDB's reference data of
+20M+ PURLs. PurlValidator also provides decentralized libraries for offline
+use that can be integrated in multiple tech stacks for all major ecosystems,
+beyond what is already available for PURL tools. The goal of this project is to
+provide an accessible, single source of truth to the security and SBOM ecosystem
+at large and improve the quality and accuracy of PURLs in use, imperative for
+CRA compliance.
+
+
 Documentation overview
 ----------------------