Asta is an anomaly in the world of Black Clover, the reason being that he was born without any mana. Instead of being able to cast spells like others, he uses Anti-Magic, energy that cancels out mana. It's unusual to be sure, but it's all because of his mother, Richita.
Richita was born with mana like everyone else. However, she was also born with the uncontrollable ability to absorb the mana and even life force of others. Because of this ability and her lack of control over it, she sucked away Asta's mana from the moment he was born. She then left Asta in a basket at Hage's church, fearing that she would suck the life out of her baby.
Some time later, Richita found Licht's grimoire, and soon after she found a young and injured Liebe. Ridden with guild over having to abandon her own child, Richita took Liebe in. These two meeting would set off a chain of events that would lead to Asta gaining Anti-Magic. Lucifero would take over Liebe's body after finding out about the devil living with a human. Richita attempted to absorb Lucifero's magic by holding on to Liebe, but Lucifero was able to take control of Liebe's left arm and plunge it into her chest. Luckily, Richita was able to force Lucifero out of Liebe's body and seal Liebe inside the five-leaf clover grimoire before dying.
This five-leaf clover grimoire is the same one that Asta receives on his 15th birthday, and Liebe would become the source of his Anti-Magic abilities. The two now have a symbiotic relationship, helping each other and uniting to defeat strong opponents. Both are also unable to use mana, making their bond that much stronger. Liebe was born with the ability to use Anti-Magic and this, combined with his lack of mana, is how he was able to escape to the living world in the first place.
Had Richita not been born with her ability to absorb mana and life force, Asta would have been born with mana and been able to use magic like everyone else. She wouldn't have been forced to live away from everyone else, and wouldn't have found the five-leaf clover grimoire or Liebe. The entire story would have unfolded entirely differently with this one change.
Given how rare Anti-Magic energy is, it was vital for Richita to have that mana-absorbing ability for Asta to be able to use Anti-Magic. She needed to find Liebe and the five-leaf clover grimoire, and she needed to seal Liebe inside of it. Had she not been able to defeat Lucifero and live long enough to seal Liebe away, he and Asta may have never met and Asta wouldn't be able to use Anti-Magic. Everything comes down to this one woman having this one specific ability and not being able to control it, and finding that one specific devil child. Talk about a butterfly effect.