[Why we learn gemara has been extensively covered on MiYodeya, e.g., here, here and there].
One of the goals of learning is to know the entire Torah (kol ha Torah kula) (see here on MY for sources whether or not it is an obligation or simply a worthy practice). Since the Torah is infinitely broad and deep, this is impossible, but learning Mishne Torah is one way to approach it.
As you know, the Rambam codifies all halachot, both those practiced today and those that will be practiced when the Beit Hamikdash is rebuilt. As such, it is broader than most traditional halachic codes (e.g., Shulchan Aruch, Kitzur SA). The language of the Mishne Torah is also exceptionally clear and structured; its organization makes it easier to remember what is where.
It is for these reasons the last Lubavitcher Rebbe instituted a daily learning schedule for Mishne Torah
It gathers all of Jewish law in a concise and clear fashion. Every
individual is commanded to study the entire Torah, a goal not within
reach for most people. However, it is possible to study the whole
Torah as compiled by Maimonides.
To get to practical halacha, I note that recent editions of the Mishne Torah (e.g., Koren's from R Steinsaltz, or R Touger's available online in English) bring rulings from Shulchan Aruch and other aharonim where they differ from the Rambam.