Someone in my shiur asked me today why it was necessary for all these amoraim to add on tefilos after their davening. Anshei k'neses ha'gdola instituted the nusach of davening. Why did these amoraim feel that it was insufficient and necessary to add to it?
I found that the Tzlach actually asks this question. The gemara on 28b says that tefillah cannot be "kevah" rather it must be רחמים ותחנונים. The Anshei K'neses Ha'gedola instituted a nusach of tefillah but this nusach is generic and not personal. It has a tendency to lead one to daven out of habit rather than heartfelt requests of Hashem. Therefore, the gemara teaches us that everyone must personalize his tefillah to ask Hashem for what he needs and not just leave it as the generic tefillah of the anshei k'neses ha'gedola.