I too find that in the beginning the Mantra was a way to keep the conscious mind busy ... however, I find, and other may find over time that the Mantra is not necessary, especially if one can obtain great 'concentration' and be totally involed into the material.
Mantras can also be used to not only keep the mind busy, but to 'program' it using whatever positive affirmations you wish. I tend to repeat Re-lax, Re-lax over and over sometimes to calm the body and mind! Do whatever works for you to make you comfortable in the process!
Just watch out you don't become too relaxed! I've gotten into trouble this weekend because I'd gotten too deep into concentration, wife was holering that breakfast was getting cold! Only after 3 or 4 attempts did I hear her ... when I did manage to get to the breakfast table, breakfast was not cold, but the wife was!