Liquid and granular fertilizer are both beneficial for the lawn in their own way. Granular is a slower feed with less risk of burn, but liquid fertilizer gets taken up by the plant faster and does not require water to be activated.
The second round of fertilizer is usually applied as a liquid because it has a liquid weed control mixed in. The combination of the two makes for an easier more efficient application.
Liquid weed control is the only way to kill weeds.