If a man wears a four cornered garment it is required to place tzitzit on it. Today many men have the custom to wear a tallit katan throughout most of the day.
If they are already wearing a tallit katan, what does donning a tallit gadol during Shacharit accomplish (as I see many of them doing)?
Perhaps the main function of the tallith gadol is as a beged meyuchad l'tefilla (a garment designated for prayer), as a sign of respect when we stand before the King (see e.g. Orach Chaim 91:2; see also Taz Orach Chaim 21:3 cited by the Aruch HaShulchan OC 21:6 who for this reason disallowed its wearing into an outhouse; See also Taz OC Yoreh Deah 283:3 who reversed this position [in alignment with Shach YD 283:6]. The Aruch HaShulchan [op. cit.] and Mishna Berura [21:14; 610:18; cited here], however, both rule stringently.)
Additionally (as @DoubleAA noted in a comment), assuming the tallith gadol is (as its name implies) larger (and without any large hole in the middle), as well as wool, there is an increased likelihood that it fulfills all the halachic viewpoints regarding a garment that can be used to fulfill the (biblical) commandment of tzitzith.
The concept is to fulfill all opinions concerning the concept of being completely wrapped (עטיפה גמורה) by the talit. This is being wrapped at least at the time of the blessing completely around the head and the majority of the body, after the custom of the children of Yishmael. This subject can be found, for example, in Shulchan Aruch HaRav, Orach Chaim, vol. 1, Hilchot Tzitzit, 8:1,6-8. The talit katan is not large enough to do this.
There are other reasons relating to beautifications of the way the tzitzit are tied, but this is not the primary reason.
