The depiction of the emperor in his military uniform particularly emphasizes his role as Imperator (see p. 112 in Varner, ed., From Caligula to Constantine, Tyranny and Transformation in Roman Portraiture.). Trajan was known for personally commanding his army from the frontlines. His military successes increased the expanse of the Roman Empire, especially along the Danube with his Dacian conquests. A small-scale portrait, such as this example, would likely have been displayed to declare the owner's support of the emperor and his military campaigns (op. cit., idem).
For small-scale portraits of the emperor in stone see pls. 39-41 in Megow, Kameen von Augustus Bis Alexander Severus.