When we initially start reciting a mantra, rules are very important. It is all about meeting a specific count (generally 108, 300, 1008) every day at the same time (generally early morning, noon, evening or midnight), same place and facing the same direction. It is purely number crunching. Hitting the target without missing is all that matters.
Once we practice this for at least a few months, the mantra begins to awaken in the body and mind gets quieter, the bliss of the mantra and the energy of the deity begins to flow through. At this point, we can go easy on numbers and focus on fine tuning the mantra - such as sync the mantra with the breath, recite the mantra in the chakras, recite with spatial awareness and other such refinement techniques which will propel consciousness from “supported awareness” to “unsupported or effortless awareness”.
For the first stage, rosary is helpful as its physical. Also, over a period of time, the seat used, the rosary used - begin to accumulate a field of energy and touching it starts adding value to the mantra session. That is not to say this is a must - I had a teacher once in Nepal who would refuse to possess anything including a rosary. He would just use fingers to count, and I’ve seen many doing fine with just fingers for counting.