If it is true [that this new prophet will not change any of the laws of Judaism] then why does it say in the Torah (Devarim 18:18) "I will establish someone like you (Moshe) for the [Jewish people] from among their midst?" [G-D is] not bringing forth [a new prophet] to make a [new] religion, only to [remind the people] of the commandments of the Torah and to warn the nation not to transgress them. Like it says in [the book] of the last [prophet] (Malachi 3:22) "Remember the Torah of Moshe, my servant." Additionally, if the [prophet]commands us with regard to a matter [that the Torah does not speak about], for example go to such and such place or do not go, or wage war today or do not wage war, or build this wall or do not build it, then it is a commandment to listen to him and anyone who transgresses his command is worthy of the death penalty from Heaven, for it says (Devarim 18:19) "And it will be the man that does not listen to the [new] prophet's words that are spoken in My name, I will interrogate him."
The Rambam now tells us the authority of the new prophet and his use. A new prophet can not erase what had been previously established, he can only add additional ancillary commandments that, according to the Rambam's examples, are for that specific time and situation. This is keeping in line with the commandment that one is neither allowed to add or subtract from the Torah (Rambam Safer Shoftim in hilchos mamarim). The Rambam continuously makes this point to show that a true Torah observant Jew could never even consider christianity or Islam as this would require someone to believe that a later prophet is coming to essentially change the previous commandments. No matter what these "prophets" do, a Torah observant Jew can not believe them. The new prophet could perform great miracles or have convincing arguments, but nothing can change the previously established commandments.
This idea brings up one of the Rambam's main ideas (Yisodei Hatorah Perek 1 Mishna 11) "[G-D] never changes, since there is nothing that can cause Him to change." The idea that a prophet would come along and tell the Jewish people that certain commandments in the Torah are no longer to be followed would go against this basic concept of the Rambam. G-D does not change, therefore, it is impossible within the Rambam's conception of G-D that the religion that G-D created could experience any permanent change. It must be that when the Torah was initially given by Moshe, it was complete and would never be significantly altered.
The main utility of a new prophet is to remind us of the true Torah laws and that we should keep them. The additional use is that they can help guide us in matters of current events that are transient in nature like whether we need to build up protection for a future assault or if that will be seen as an aggression (build a wall or not), avoid a specific area or leave a specific area because of a future calamity (do not go to a certain place or do go), or perform a preemptive strike on an enemy or not (wage war or not). These concepts would not be found in the Torah as they are only appropriate for a specific time period and the Torah only contains that which is eternal and not bound to specific time periods. For example, war strategy against the Romans is only relevant against the Romans. Once that kingdom is gone or before they exist, there is no reason to speak of them. However, the commandments about sacrifices are relevant in any generation that has a Temple (Beis hamikdash), that is relevant in the first temple period, second temple period and it will be relevant again during the third temple period.