All great literature has basic roots in the expression of the human spirit. Each bud becomes a leaf: a new life in itself just as each writer produces their own text. Great writers' work may compete for the light of fame, as leaves on the tree, but the tree of literature is at its best when redolent with leaves, flowers and then fruit. Each season has its beauty just as each age has its literature. There is change, but also continuity.

Writers, especially Shakespeare, address universal truths about being human and reveal inspired insights (or as Renaissance writers thought, divine truths) that illuminate the heart and mind leading to love and virtue in behavior. This, if done as Shakespeare, Spenser, and Sidney did it during the Renaissance, provides safety and protection against some of the storms of life (learn from Lear and Rosalind and follow their best examples, not their worst) and fosters human growth because truth and good are explained and exemplified while villainy is illustrated and warned against.

Pablo Neruda's metaphor is a fitting one, and here we are talking about it, and still basking in the shade of that tree.  Great writing inspires more great writing; I don't believe there is probably a single author, published or not, who hasn't been inspired by some other author, play, story, or poem. 

Writers help others grow intellectually and emotionally by showing readers new ways to see the world and to see themselves. There is an enrichment of perspective offers in literature. 

Writers help other writers by showing them what a person might choose as a subject. Every great book, though unique, is also a guide showing other writers how to express one's ideas on subjects of love, fear, parenthood, etc.  

This is a new analogy for me, but I really like it! Writers allow for cultures to be sustained and spread by recording the ideas and knowledge and history of a people, in the same way that leaves use photosynthesis to insure the survival of a great tree. Leaves protect birds from storms or heat in the same way that writers may raise public awareness or opinion through their writings, may support the distribution of information that is needed or desired as cultures interact with each other, may save people from repeating foolish acts that have been unsuccessfully tried before.