The carboxy-terminal domain is also the binding site of the cap-synthesizing and cap-binding complex.  In eukaryotes, after transcription of the 5 end of an RNA transcript, the cap-synthesizing complex on the CTD will remove the gamma-phosphate from the 5-phosphate and attach a GMP, forming a 5,5-triphosphate linkage. The synthesizing complex falls off and the cap then binds to the cap-binding complex (CBC), which is bound to the CTD.
The 5'cap of eukaryotic RNA transcripts is important for binding of the mRNA transcript to the ribosome during translation, to the CTD of RNAP, and prevents RNA degradation.