For the most part, unless an author provides a note upon why a text was written (as Mary Shelley does with Frankenstein) one can only assume why any text was written. Critics will argue, analyze, and scrutinize a text in order to define an author's intent behind its creation and construction. This is called author intent. As stated, some author's define their intent for readers; others do not.
In regards to Emily Bronte's poem "Remembrance," the title is the only clue readers are given in order to discern Bronte's reasoning behind writing the poem. The poem tells of Queen Rosina Alcona's lost love, emperor Julius Brenzaida. The poem's form, an elegy, is meant to lament the loss of a loved one. The poem depicts the imaginary world (a paracosm) of the Bronte sisters called Gondal. Therefore, the poem could be a remembrance of the world and the time Emily spent with her sister "in" Gondal, or the poem could be a remembrance of a couple of the characters created by the sisters who lived in Gondal.