This app uses common words that children would hear often and that would be useful to know. Many times, programs have children memorize sight words like the, from, and said - but these words don't have an easily discernible meaning like the ones chosen for this app.
As children are learning about letters and letter sounds, it's important for them to simultaneously learn that letters are used to form words and that words carry meaning. Memorizing words is a much bigger part of reading than people realize. Once a word is memorized, it doesn't need to be sounded out.