It can take years for an I-130 to be approved so that you can join your partner on an immigrant visa in the US. The K3 visa is quicker to apply for and means that you can join your partner in the US whilst you await approval of your Form I-130.
However, in some cases your I-130 may be approved before you can make your K3 visa application. In this situation you will no longer be eligible to apply for the K3 Visa and instead must go straight to applying for your immigrant visa.