What is the need and purpose of packet injection within WiFi attacks

According to my research, the most common WPA/WPA2 WiFi attack requires a chipset capable of packet injection.

However I am not sure what this is, and what purpose it serves once you have the capability to inject packets.

I thought that all WiFi chipsets could send/receive data, and assumed packet injection would come under the sending protocol(whatever that may be) - but I must be missing something as only certain cards can apparently inject packets.