The ceratopsian jugal horn is unique in that it comes in two distinct forms: a non-capped form, where the jugal itself flares outward to a point without the assistance of an epijugal ossification; and a capped form, where the jugal flares outward to (usually) a point and is also capped by a separate epijugal ossification, sort of like the nasal horn. What's especially interesting is that the most impressive jugal horns--non-capped forms--are found in the most basal ceratopsians. The crowded genus Psiattacosaurus includes three species with particularly impressive jugal horns: P. sibiricus, P. major, and P. gobiensis.
In P. sibiricus, the jugal horn width (from tip to tip) is greater than the length of the skull. This Russian genus also has the distinction of being the horniest (snicker) psttacosaur. In addition to its impressive jugal horns, P. sibiricus also has small postorbital horns and amazingly large palpebral horns. I would not want to mess with P. sibiricus!
P. major is the largest species in the genus and features a large, triangular jugal horn which, unlike P. sibiricus but like most other ceratopsians, points lateroventrally. While not as wide as the skull is long, the jugal horn width is substantial, and a good swing of P. major's head could cause some serious damage.
Finally, P. gobiensis, the most recently-described species of Psittacosaurus, has large, laterally-pointing jugal horns that are textured in such a way as to suggest a large keratinous covering. Interestingly, P. gobiensis' jugal horns curve gently downward along their length, unlike the straight jugal horns of P. sibiricus and P. major. Among ceratopsians, these species have (so far) the most impressive jugal horns!
Epijugals didn't pop up until later in coronosaur evolution. Protoceratopsids (like Protoceratops, above) have large, flaring jugals with rounded ends and similarly rounded, blade-shaped epijugals. They give the skull a unique look in anterior view.
Ceratopsidae never developed the jugal "spikes" of psittacosaurs or the flaring jugal blades of protoceratopsids. Focusing instead on postoribtal and/or nasal horn development, most ceratopsids simply forgot about their jugal horns. In almost all specimens, the epijugal quickly fuses to the underlying jugal bone, and the two grow outward together. The separation is barely apparant in older animals. Just one chasmosaurine, Pentaceratops, has managed to somewhat revisit the glory days of jugal horns. Pentaceratops has a jugal horn length of about 144 mm. That may seem huge, but keep in mind that that's 14.4 cm, or about six inches--nothing to brag about when your postorbital horns are 56 cm long (about two feet).
Jugal horns are not phylogenetically relevant unless you're talking about the separation between capped and non-capped forms, which seems to be a consequence of higher taxonomy (coronosaur or non-coronosaur). Among ceratopsids, the jugal horns are almost always short and unimpressive, and all those illustrations you see of Triceratops with whopping jugal horns are almost certainly inaccurate, unless the epijugal was, itself, capped by a massive keratinous cover, which I find unlikely. The large jugal horns of protoceratopsids may have had some role in sexual display or species recognition (or sexual dimorphism), and the retention of epijugals was probably inhereted by ceratopsids, but that group never really did anything with them. They may be vestigal structures, overshadowed by the other horns and spikes of the head.