Do you want to write your melody although you may not read music well. Many a musician can play in a rhythm section -- does not read much music -- but learns and plays harmony to the melody by blending among other instrumentalists by ear (improvising).
- The notes of the melody are not the first consideration for strumming, picking, and arpeggio piano chording kind of musician... who is in key and harmonizes to the key, but is not really "clued" to knowing the exact melody line or harmony theory...
- One may be a good recording session player or band member par excellence by knowing chords inside and out without reading the sheet music, ie: for purposes of commercial/professional entertainment or church music!