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The main function and advantage of these translucent plastic tubes is to dim available sunlight, except at the top, which encourages the tree seedling to put energy into vertical growth. The tube forces the tree to grow above the reach of browsing deer, while also leaving behind ground-level shrubs, forbs, and grasses that compete for light. Once the seedling emerges at the top of the tube into full sun, it then branches out and begins to form a full crown.

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Tree tubes provide other advantages. They also protect seedlings and saplings from being rubbed by bucks, and they offer some protection from harsh weather conditions, while also conserving moisture around the seedlings. Two different scientific studies, both conducted at Auburn University in Alabama, found significantly increased survival (nearly double in most cases) and vertical growth (double and sometimes triple the growth rate) in seedlings protected with tubes compared to non-protected seedlings.

Information from  qdma.com/tips-using-tree-shelter-tubes/