The first reason for learning French is its ubiquity. French is an official language in 33 countries, and, like English, is spoken as a native language on 5 continents. It is also an official language of the United Nations, and of the Olympic Games. This alone attests to its importance, and consequently to its usefulness.
The second reason for learning French is its rich contribution of literature. Imagine having to read translations of Les Chansons de Roland (c. 1100), Le Roman de Renart (c. 1200), the poems of François Villon (c. 1456), Gargantua and Pentagruel : by François Rabelais (1532 - 1552), the Essays of Montaigne (1571 - 1592), the great playwrights of the classical period : Molière, Racine, Corneille (1600s), Les Principes de la Philosophie : by Descartes (1647, from Latin to French), the Fables of Lafontaine (1668 - ), Candide : by Voltaire (1759), the novels of Balzac : his "Comédie Humaine" (1819 - 1848), Les Fleurs du Mal : by Baudelaire (1857), Proust's novel in 7 volumes : À la recherche du temps perdu" (1913 - 1927), not to mention works of Albert Camus and Jean-Paul Sartre (mid-20th century)!
This brings me to the third reason for learning French : reading the literature in its original language, you experience the culture in a special way. For example, the very popular expression "revenons à nos moutons" (let's return to the subject of sheep) comes from La Farce du Maître Pathelin (1400s) in which the judge, exasperated, tells Guillaume, who has been cheated of fabric and robbed of his sheep, and who confuses the two matters, to return to the subject of sheep. So, anytime you want to get back to what you were speaking about, you simply say: "revenons à nos moutons".
The fourth reason for learning French is music. It is very important in opera, with composers like Rameau, Berlioz, Bizet and Debussy. French opera had its beginnings during the reign of the Sun King, Louis XIV (late 1600s). The 20th century brought us the music of Charles Trenet, Charles Aznavour and Edith Piaf. Today, French music is still very popular and enduring, as with the ballad of Barbara "L'Aigle noir" and the very contemporary sound of "Petite Soeur" by Laam.
The fifth reason for learning French is that it will help you to understand your own language better. We learned our own language by immersion, and take it for granted. Learning French will force you to look at your own language more closely because your approach will be more conscious, more cognitive. For example, in English we say "the apple I gave you is red" - we can omit the relative pronoun "that" after the word "apple". In French, we must use it - "la pomme QUE je t'ai donnée est rouge". Another example regarding spelling is the word "responsible". Some may be apt to misspell this word by putting "a" in place of the "i" ; this would be the French spelling. Thus, in learning to spell French correctly, you hone your skills in English spelling as well.
The sixth reason for learning French is that it will give you insight to Latin, the language from which it has evolved. Incidentally, although English is a Germanic language, over half of its words are of Latin origin. Thus, learning French should be all the easier for the English speaker, and all the more if your language is another Romance language. For example, in English we have the word "puerile" ("puérile" in French : juvenile, childish), which can be traced back to the Latin word "puer" (boy). This insight into Latin will facilitate understanding other languages that draw from it.
The seventh reason for learning French is that it is a beautiful language. French is known for its nasal sounds, and the uvular "r" (pronounced at the back of the throat, as opposed to the trilled "r" of Spanish). There is yet another sound that can be challenging for English speakers, the "u". However, these are the sounds that account for the euphony of the language. There is yet another factor that is indispensable in the phonetic charm of French : its elisions (the linking of consonants and vowels of adjacent words). For example, we say "vous avez" (vou zave, meaning "you have") ; the "s" in "vous" elides with the "a" in "avez" to give a more graceful sound. Don't be alarmed; you will learn these things very quickly.
These are at least 7 major reasons to learn French! I am waiting to hear why you would like to learn French, and I am looking forward to speaking with you.