Extract is a compressed format of data provided by tableau for better performance when huge volumes of data are there.
extracts are better than live connections.Every database will have a live connection in place so according tome there is no DB which supports or needs only tableau data extract.
Yes many data sources will support live and extract.
Sales force and hana also support live or extract. but sometime you will get error like below.
Errors during extract
When you connect to Salesforce using Tableau, the data is automatically extracted into a Tableau Data Extract file. In some cases, certain fields cannot be extracted because of character limits. Specifically, text fields that are greater than 4096 characters and calculated fields will not be included in the extract. If you have calculated fields in your data, you will need to recreate them in Tableau after creating the extract.
I believe HANA can work as both a live and an extract.
Salesforce only works as an extract, from what I understand this may be performance and data structure related.