Views 3 can query different backends such as Solr via Search API. This project is to continue to decouple Views' functionality for the SQL backend.
The problem with the current setup is a lot of Views is still geared towards a SQL backend which means field handlers can't be reused which ends up causing less functionality and duplicate code. Also a view built for a SQL query uses completely different field handlers to one for the Search API so a site builder wanting to upgrade a View from the SQL query to a Solr query has to recreate it from scratch.
Aim as User Stories
As a site builder you want to be able to build a View on your SQL Database and later move it to another data source such as Solr, MongoDB or Elasticsearch
Different backends will support different types of queries, and the site builder will need to be informed of what features will be degraded.
Indexes provides by Search API may not be setup to index a field that a user wants to add their view - it would be nice for the index to be extended as part of the views building workflow.
Developers creating Views query classes for different data sources want to be able to use the existing field handlers
The field handler provide field formatters the site builder is familiar with and may already have configured on a SQL backend.