These tube are installed to encourage the formation of deeper roots, by delivering air and water lower down. This is advantageous for the tree, when drought conditions occur.
They are an unintended consequence though:
It is often used as the way to water trees, to the exclusion other methods. The person watering is of the belief that when the pipe is full, the tree is watered. WRONG.
When new trees are planted 100% of their roots are within the rootball. It is these that should be watered, not the soil around and below the roots.. Watering through the pipe waters these areas only.
For the first year 80% of the watering should be undertaken slowly over the top of the rootball. The 20% should go down the pipe. The watering should be undertaken to avoid runoff.
As the tree establishes the area around the rootball can be watered to encourage the roots to spread out.