Song writing rules and guidelines have benefits. Instead of creating a varied mass of notes and music, rules offer a path to follow (not obey) to create a neater song, with better lyrics and more organized music. Following rules can help a beginning composer write a first great song, or aid a musician unblock during a difficult time in songwriting.
Song writing involves different aspects. Composers take notice of form and flow--where the music is coming and going, and how it moves along that path. Meaning and theme take part in every aspect of a musical piece, and are kept in mind for every portion of the song. Repetition, verse and chorus are all part of form, which vary from one musical style to another.
Truly creating a piece of music that holds meaning to you is more important than following a set of rules, but understanding the different forms, functions and theories adds great insight into personal composition. Great artists know different painting styles; famous writers read plenty of books, in different genres and writing forms; and musicians and composers understand the different forms, styles and features of music and how it is written. Classical song writing form and rules differ from jazz and contemporary, and understanding the rules for songwriting gives you more knowledge to get away from these rules and create a personal, even experimental song.
The belief that song writing is a scientific or logical process is not completely true. Though composers take logical consideration at times, music is an art and should be treated as one. Creativity is more important, and following rules can be dangerous if it inhibits the passion and artistic aspect of the song.
Aaron Copland, a musician and composer, wrote many books on composition and song writing rules, and they are used today in curriculum for composers studying music and its form, style and function. Reading Copland's books, especially "What to Listen for in Music," will add great insight from a renowned expert in songwriting theory and form. Even though Copland lived and wrote his books in the first half of the 20th century, his discovery, knowledge, ideas and compositions are still respected and used today.