The DB Connect app allows Splunk to read, index or otherwise use actual Database tables, views and queries directly. So for instance if you had your asset list inside some other system that had a DB you could get to, you could use Splunk to read that table into itself for use there, or use it directly as a lookup from Splunk.
Using a forwarding on the DB host gets you their logs, events, and occasionally other information - mostly from the OS level although that's a little blurred because many DB logs are also os-level logs. But it doesn't really allow you to read data from the databases. (Unless you have a job in your DBMS that runs and dumps information into a file on a schedule, you could then use the UF to read that and send it to the indexers).
The various apps and add ons (like this one for SQL Server) is where you get the DB logs that aren't "OS-level" as I mention above, and which allow you to do magical things with the management layer of SQL - collecting audit trails from SQL, or detailed performance information for SQL.
Does that help?
