You only use it to meet the spending requirements for the signup bonus. For example the Slickdeals home page has the Chase Sapphire right now: "50k Bonus Point

You only use it to meet the spending requirements for the signup bonus. For example the Slickdeals home page has the Chase Sapphire right now: "50k Bonus Points ($625 towards travel) w/ $4000 Spent in First 3 Months of Account". Other cards have similar spending requirements.