mtalk.google.com is needed by GCM (cloud messaging). For those using adblockers it was always recommended to whitelist it (not block it), otherwise one would have problems with push messaging. If Device Registration and GCM are turned off then it doesn’t matter.
Let’s not forget the purpose of microG. 
Someone did raise that question a long time ago at the microG GitHub…
Why does GCM needs to connect to mtalk.google.com ? · Issue #569