I just curious, why we need worklight adapter to communicate with the database? Why not can't using direct ajax? Last time I'm using direct ajax it's not working at all. After I switch to worklight adapter it's running like charm? No problem at all. Why?
You did not provide any specific details about your scenario, like Where is that backend system located, if there is any security involved, what is the topology, etc... so why it's not working with AJAX is impossible to answer.
As for why use Worklight adapters, read:
http://javatechig.com/cross-platform/ibm-worklight/how-to-create-a-http-adapter-in-ibm-worlight
Adapters provide templated (adapter types) connectivity settings to various backend systems - HTTP, SOAP, SQL, JMS and so on