One has both to retain the whole message in memory and keep track of where one is up to in production of the message. To find out just how hard that is without feedback, try writing a letter in the dark, with multiple interruptions and distractions such as those found in any classroom or social gathering. The memory demands on people who use communication aids are enormous. Steven Hawking 10 minutes and Anne McDonald 17 minutes. To get an idea of what this means, speak into a tape recorder for a minute then come back fifteen minutes later and repeat exactly what you said previously, without hearing the tape again first.
People using speech output communication aids receive primary aural feedback.
People who use letter boards receive aural feedback only if their partners say the letters, words and sentences they spell aloud. They only receive visual feedback if their partners write down the letters they select where they can see them. People with poor eyesight need auditory or tactile feedback, either from their devices or from their communication partners. People who can see and who find auditory feedback disturbing either need devices with displays or need their facilitators to write down each word they spell. Don't think feedback is just for novice spellers or novice facilitators - it is essential for every communication aid user.