For long term improvement, you need to have multiple revolving lines of credit. It moves you out of a "thin" scoring bucket.
Adding CL to an existing card is not as beneficial. FICO scores % util of CL, and not CL. Increasing your CL wont help, if you still maintain the same % util on the card.
The period of credit rebuilding is the time to take the hits for new inquiriies, and lowering of your average age of accounts, for their potential future benefit.
