Riojasaurus, like other prosauropods, is built uniquely in the dinosaur world. It is almost a mix between the bulk of the sauropods and the flexibility available in hadrosaur bodies. While the long neck is certainly much more sauropod than hadrosaur, the agility of the body is something that, while sometimes portrayed in animals as large as sauropods, is a much more believable, all the time, characteristic in smaller animals. The long neck would have most definitely helped gather food for the dinosaur and its battery of teeth, which we now know a good deal more about since at least one well fossilized skull has been unearthed, were most definitely meant for ingesting plant matter at a high rate, though not so much for chewing as for cropping and clipping then doing just a bit of mashing, not like the teeth found in the cheek area of triceratops, for example. These teeth were leaf shaped with 5 at the front of the mouth and 24 more behind, all of these were reportedly found in the upper jaw as well.
That skull that was found had interesting ridge details not unlike those found in theropods, and the overall shape was somewhat elongated. However, if prosauropods did in fact give rise to sauropods, remember that there is some discussion that both had a common ancestor and convergently evolved rather than one from the other, then the snout could have shortened and the dental battery become more compacted into the leaf clipping and swallowing set ups that we typically see in the larger sauropods of the middle to late Jurassic and early Cretaceous. The skull, regardless, is a very important piece to the puzzle and whether or not all of the teeth are preserved or it had lost some teeth, the overall shape and makeup of the skull tells us a lot about this dinosaur.
The use of the neck in foraging could have been conducted in two ways. In situation one we have the body being used in the hypothetical sauropod/realistic hadrosaur manner in which we would find Riojasaurus reared up on its massively built hind legs reaching into the upper foliage with the strong forelimbs and foraging as high or as far into the trees as the long neck and forelimbs would allow it to do. This is plausible, even in some sauropods I imagine, for Riojasaurus as it did not have too much bulk to rear up and also had rather large and well built hind legs for a dinosaur of the Late Triassic. The second way that these animals could have fed is in a manner more in line with the Nigersaurus, Triceratops manner of feeding which would mean spending the majority of its time with its head down below the shoulder blades finding low lying shrubbery, grasses, and ferns. One problem in the last sentence that sticks out right away is grasses. There weren't any grasses for it at this time in Earth's history. That means that the available food sources down along the ground at this time, the Late Triassic, would have limited Riojasaurus to the ferns that grew along the ground and some types of shrubbery. Picking one or the other mode of foraging would potentially severely limit the diet of Riojasaurus. It makes sense, then, that Riojasaurus actually used both methods of foraging to find food. Ferns, shrubs, fern trees like the cycads, would have been available whether the dinosaur was foraging high or low and as one of the largest herbivores of its times it could have easily showed its dominant right to whatever food source it choose at whatever time.