The entire purpose of this build revolves around these two facts:

1. The utility & sustain from tear enabling you to spam your W whenever it's up from CD is severely under-rated.
2. Muramana once completed is a strong item on Kayle.

But essentially it's all about the first point. Your W at rank 1 and zero AP heals for 55 and grants you 24% movement speed. Having a short little 24% movement speed burst whenever you go to dodge an enemy skillshot is incredibly helpful. If you don't have tear, you can't spam your W in-lane without quickly running out of mana. But with tear, you don't need to exercise near the same caution. Yes, you can still run out of mana, but it takes a long time.

In addition, while small, the heals add up when you can use W over and over again.

I don't think my words are doing this justice, but the utility and sustain gains tear grants you are way more than people expect.

I just achieved Emerald 4 playing mostly top. Kayle, Nasus, Master Yi, and Jax were my top champions, and I played Kayle substantially more than the rest. In total, I had a 68% win rate over 98 games on Kayle with this build.