How did Harry get the Philosopher's Stone

Why did Voldemort try to steal the Philosopher's Stone?

The ritual was his backup plan after knowing he couldn't get the stone.

The Dark Lord's original plan, and the plan he preferred, was to steal the Philosopher's Stone. He first decided on other methods, after this he knew there was no chance he could get her.

“There was no longer any hope of stealing the Philosopher's Stone because I knew Dumbledore would have seen it destroyed. But I was ready to resume mortal life before chasing after the immortal. I have set my goals lower ... I would be satisfied with my old body and strength again. " - Harry Potter and the Goblet of Fire, chapter 33 (The Death Eaters)

It is clear that he considered the stone, which also offers immortality, superior to other methods of restoring himself that did not assure him of immortality.

He was also stronger and more capable before the ritual than when he owned Quirrell.

Even if the Dark Lord used unicorn blood and owned Quirrell, he was very weak.

"I was torn from my body, I was less than ghost, less than the meanest ghost ... but still I was alive." - Harry Potter and the Goblet of Fire, chapter 33 (The Death Eaters)

Before performing the resurrection ritual, he had already regained some of his strength and found himself in a rudimentary body of his own instead of having to rely on property. He and Wormtail used a potion of unicorn blood and venom from Nagini (a type of snake that he may not have access to while having Quirrell in ghost form) to strengthen it and give it its own shape, if not a complete one.

“He was the able servant I needed, however, and although he is a poor wizard, Wormtail was able to follow the instructions I gave him which would lead me back to a rudimentary, weak body, one that I would be in to be able to live while I wait for the essential ingredients for a true rebirth ... a spell or two of my own invention ... a little help from my dear Nagini "- Voldemort's red eyes fell on the ever circling snake -" a potion made from unicorn blood, and the snake venom that Nagini provided ... I was soon back in almost human form and strong enough to travel. " - Harry Potter and the Goblet of Fire, chapter 33 (The Death Eaters)

He was then stronger, could wield a wand, and had his own shape that could be placed in the cauldron to bring him back to a proper body.

Bellatrix

@TGnat The Philosopher's Stone assured the Dark Lord of Immortality that the Resurrection ritual did not. He tried to steal the stone because he believed it was the best way to get what he wanted - certain immortality.

tonysdg

+1. This also explains Big V's later obsession with the Deathly Hallows. Since he could not cheat death with the philosopher's stone, he planned B - cheat death with the sanctuaries of death.

DariM

@tonysdg Voldemort had no plans with the sanctuaries. He wanted the Elder Wand because he believed he was more powerful and that he would overcome the twin core problem. But it becomes clear that he doesn't even know or use the other sanctuaries. His horcuxes are his method of cheating death, and they are still in tact (until late in Deathly Hallows)

Frollo

From the last quote, Voldemort also says that the rebirth rituals required "a spell or two of my own invention." He does not state when he created these spells (or how much time it took to create them), so it is entirely possible that the ritual was not possible at all at the time of Harry's first year due to the lack of spells.