Learning to read musical notes and to play straight from them is often found to be a challenge by beginners and, in particular, young players. Writing the note names underneath the actual notes might help at the initial stages to start playing the piano. However, it takes skill, time and effort to write the note names down for longer songs.
This is when books with the letter names already written down are nice to have. This also ensures that the names are correctly written down. For inspiration and ideas on what books to use, see the link above fr some excellent resources.