Mantras and Japa help us to concentrate. We live in a world that is overflowing with sensory pleasures and stimulation. Our lives are very busy with work, errands, chores, etc. Therefore, it is very difficult to simply still the mind. Mantras and Japa and special pujas serve as bridges between this world and the divine realm. They offer us a way to transverse the water which may seem impassable.
However, mantras and japa are means to the end of God realization. They are the means, not the end themselves. Once we develop that close, intimate bond with God, mantras and japa become less necessary.
Imagine that you love someone with all your heart. You don’t need to take a ‘mala’ and recite her name over and over gain to remember her.
Similarly, once we have that deep love for God, we don’t need to continue doing japa to bring us into contact with Him. We will be in contact all the time. Our lives will become our japa.