By the context of your statement, I'm guessing that you're just looking at the 5e Monster Manual lore? This is inconsistent with previous D&D versions, as noted above (in previous versions, rakshasas were not created in the Nine Hells). However, if you're just looking at 5e, then perhaps the answer is obvious. The MM states that rakshasas are formed from devils who shed their skins and project their spirits into new bodies, so as to better infiltrate the Material Plane. Hence, prior to becoming rakshasas, they are simply devils that already fit into the established hierarchy. Some of them were soldiers in the Blood War, some of them were bureaucrats in the infernal hierarchy, some of them were agents of the Lords of the Nine.
The 5e MM also states: "For a rakshasa, death on the Material Plane means an agonizing and torturous return to the Nine Hells, where its essence remains trapped until its body reforms- a process that cart take months or years. When the rakshasa is reborn, it has all the memories and knowledge of its former life, and it seeks retribution against the one who slew it." Note that it doesn't state exactly which body reforms. Perhaps the rakshasa reforms in its original devil body... which could be a pretty amazing story hook for a campaign. The party takes out a CR 13 rakshasa... and then some time later, they get ambushed by a very angry CR 20 pit fiend, who seems to be nursing a grudge...
Alternatively, maybe the diabolic spirit reforms in the Nine Hells in its new rakshasa body. In this case, it's probably a figure of some derision and amusement for the other devils. Think about it... you've just shed your diabolic body to create a new rakshasa form on the Material Plane. Your existence back in the Nine Hells in rakshasa form implies that somebody killed you, and you've recently regenerated. It's like wearing a big sign saying: "Some humans killed me!".
And for those rakshasa who willingly travel to the Nine Hells (using their daily plane shift ability), their motivations for returning are likely the same as any expatriate citizen. Some may be visiting because they're homesick; others for business; others because they have personal holdings (property, slaves, wealth) in the Nine Hells that they wish to access. Maybe, in particularly rare cases, they've returned to the Hells because of bonds of friendship or even "love". There was a classic Planescape adventure that had a Romeo-Juliet theme between a horned devil and a succubus (which were demons, back in those days). I think there could be an equally interesting story to tell about a devil that shed it's skin to become a rakshasa on the Material Plane... and is now seeking adventurers to help escort it back to the Nine Hells to visit its erinyes devil ex-lover, while avoiding the vengeful rage of other rival devils who despise that their old "friend" managed to escape the Hells by becoming a rakshasa.
ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHy4tMCtZKKrXZZ6s63KrJ%2Baq5GoerG4wJycZqGeYsGpsYynoKedXZ2yrbjSZ2xuaGhrg3A%3D