Packet encoder

Please someone may explain that what are packet encoder/decoder? there
is no eneough explanation. I need like why we are using it, and why qpsk
demod not working without it properly?

Tom, please at least could you give me a dokument to understand packet
encoder and decoder. it is confisuing, for example psk mod/demod have
differantial encoding like packet encoder but without using packet
encoder why i cannot get my signal properly?