Levi wanted to establish an alliance with legendary knights because he wanted to rely on their strength and connections to collect more and better quality breathing techniques. They could also exchange their ancestral Bloodline Breathing Techniques with each other. Regardless of whether they could cultivate these techniques or not, they could use them as references.
Of course, Levi had some selfish motives. He certainly coveted the breathing techniques of other legendary knights. However, he truly wanted to create a supreme path for knights.
