In the brocha of we ask Hashem to "grant good reward to those who sincerely trust in Your name". Is this really necessary? Do we not have a promise that Hashem rewards for even the smallest mitzva? Do we have any reason to doubt that the tzadikim, chassidim, etc. need our tefilla to earn their reward?
Sha'ar Habitachon of Chovos Halevovos, chapter 4, writes that the reward of the next world is split into two sections:
There is a type of reward which is not given because it is strictly deserved, but rather out of Hashem's kindness.
He explains this further:
As such, it is reasonable to pray for the receiving of this reward, since it is by no means deserved and not legitimately earned.
Here we are asking that the reward be in the here and now (as this is the topic of all the prayers in Shemona Esrei). Reward in this world is far from a given, even if it will be ultimately made right later.
(For a source, see here, where he uses that to explain why the language is others praying for the Tzaddikim, as opposed to them praying for themselves - because they want their reward in the world to come).
The siddur שי למורא quotes the Rokeach as saying that the request is for a good reward in this world and the next. He then quotes the סדור יעב"ץ who says that the Good Reward in this world is opportunities to do mitzvos and Torah, and increase them, because "the reward of a Mitzvah is a Mitzvah" and this is certainly a "good reward".