Can anyone explain to me what a Pillar of Light does?

It doesn't seem to heal and it can't see very far.... so what's the point in it?

It has a healing Aura like you have already found out. It is similar to the Priest of Lihrs, but relatively less. It will have a better importance in Sages Mod. As of now, it works best behind a gate or a wall of bastions, similar to the Zhon Sacred Fire. Its not a worthless unit, but near worthless.

Verunan Pillar of Light is useful, because the Zhonian Sacred Fire is useful too. I fought once online with Henrique and he took Zhon. I was unable to destroy his Death Totem, because of the Sacred Fires and Thirsa's repairing combination. Now that I am thinking of it, when I play with Veruna, I should use Pillar of Light behind Watch Towers, Mortars, Bastions, Lighthouses, Crossbowmen, Ballistas, Musketeers and Centaurs. It would make them more durable.

The main thing is, you must need a positive mana to make it heal faster. You may see it in a case that you mana suddenly become negative about -200 or -500. That is because the sacred fire, pillar of light or the priest of lihr are working. But maybe the range of heal is higher then both, is what i suppose, not sure.

If you have a few mana spare, a single Pillar Of Light can make a unit almost immortal. I once had an Amazon Knight in front of a Pillar Of Light and it was healed in an instant. My opponent had destroyed almost everything else I have had, however he had to use a lot of Tortoises and Beast Riders firing together against my amazon in order to destroy it in the end!

IMO the healing power of the acolites, veruna acolites, sacred fire, and pillar of light, should NOT waste your own mana, they would just auto heal without mana sacrifice
But maybe this would be too overpowered..
Making 10 sacredfire healing each other with titants to boost defense hmm