He needed followers to restore his body.
Voldemort never explicitly states that he recruited followers to specifically aid in his quest for immortality. His followers are usually more relevant to his other goal of ruling the wizarding world, though they have occasionally been of some use to his goal of immortality. Though he never stated that he all along had planned to recruit followers for these purposes, he does mention a few uses they would have in the specific instance that he was ripped from his body and needed to restore himself to one. When he was bodiless, which left him unable to do the magic he required to do so himself, he expected that one of them would find him and restore him to a body.
“Nevertheless, I was as powerless as the weakest creature alive, and without the means to help myself … for I had no body, and every spell which might have helped me required the use of a wand …
‘I remember only forcing myself, sleeplessly, endlessly, second by second, to exist … I settled in a faraway place, in a forest, and I waited … surely, one of my faithful Death Eaters would try and find me … one of them would come and perform the magic I could not, to restore me to a body … but I waited in vain …”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
He required the assistance of an able-bodied servant for even a return to a rudimentary, weak body.
“However, he was the able-bodied servant I needed, and, poor wizard though he is, Wormtail was able to follow the instructions I gave him, which would return me to a rudimentary, weak body of my own, a body I would be able to inhabit while awaiting the essential ingredients for true rebirth … a spell or two of my own invention … a little help from my dear Nagini’ – Voldemort’s red eyes fell upon the continually circling snake – ‘a potion concocted from unicorn blood, and the snake venom Nagini provided … I was soon returned to an almost human form, and strong enough to travel.”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
Additionally, he needed the flesh of a servant as an ingredient in the potion that would more truly restore his body.
“I set my sights lower … I would settle for my old body back again, and my old strength.
‘I knew that to achieve this – it is an old piece of Dark Magic, the potion that revived me tonight – I would need three powerful ingredients. Well, one of them was already at hand, was it not, Wormtail? Flesh given by a servant …”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
While he had a body, his followers were indeed not particularly useful to his goal of immortality, but they did become more useful to it in any instance he became bodiless.
Also, both aid his core aim of superiority.
Furthermore, both Voldemort’s goals, obtaining immortality and ruling the wizarding world, stem from the same core aim. All of Voldemort’s actions were driven by one motivation - to be supreme, extraordinary, to set himself apart from others.
“There he showed his contempt for anything that tied him to other people, anything that made him ordinary. Even then, he wished to be different, separate, notorious. He shed his name, as you know, within a few short years of that conversation and created the mask of “Lord Voldemort” behind which he has been hidden for so long.”
- Harry Potter and the Half-Blood Prince, Chapter 13 (The Secret Riddle)
The reason he wanted to conquer death was because he saw death as a shameful human weakness, in addition to his fear of it. He sought immortality to avoid succumbing to death.
“Finally he was forced to accept that his father had never set foot in Hogwarts. I believe that it was then that he dropped the name for ever, assumed the identity of Lord Voldemort, and began his investigations into his previously despised mother’s family – the woman whom, you will remember, he had thought could not be a witch if she had succumbed to the shameful human weakness of death.”
- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)
To distinguish himself to an even greater extent from the common wizarding folk, he wanted to obtain for himself a place in magical history, and become a legendary figure. Taking over the wizarding world and establishing wizarding rule over Muggles would have served to further this aim.
“His pride, his belief in his own superiority, his determination to carve for himself a startling place in magical history; these things suggest to me that Voldemort would have chosen his Horcruxes with some care, favouring objects worthy of the honour.”
- Harry Potter and the Half-Blood Prince, Chapter 23 (Horcruxes)
Becoming the leader of a world where wizards ruled over Muggles would allow him a place in magical history, as well as make him the most powerful among an already powerful group, the best among the elite.
“For years he talked of the Dark Lord, who was going to bring the wizards out of hiding to rule the Muggles and the Muggle-borns … and when he was sixteen years old, Master Regulus joined the Dark Lord.”
- Harry Potter and the Deathly Hallows, Chapter 10 (Kreacher’s Tale)
His desire for immortality and his desire for power over are connected in that they both stem from his main driving force, his need to be extraordinary, better than the others.
