Every card uses a chipset from different manufacturers. The Alfa card gets recommended all the time as it uses a chipset (Ralink RT3572 on the V2) compatible with the tools you will be using (aircrack etc).
It also has good throughput and if you want to do more than just monitor, e.g Evil Twin etc you will need two NICs anyway, so always good to have a selection.
Your built in NIC may very well have the right chipset and be compatible with the tools. Most cards can go into monitor mode it is only when you start doing more complex work that you may or may not notice issues with your internal NIC.
Another advantage of an external card is the ability to add a better arial. The Alfa comes with a 7dBi antenna as standard, good for picking up a broad range of networks close by. If you need longer, narrower range and have a direct line of sight to your target a 25dBi Yagi antenna gets good results.