What is the purpose of the ten Hail Marys in the Rosary if you should not focus on them?
To give light on your question, let us first see the definition of prayer.
In the Catholic Church, prayer is "the raising of one's mind and heart to God or the requesting of good things from God." It is an act of the moral virtue of religion, which Catholic theologians identify as a part of the cardinal virtue of justice.
Prayer may be expressed vocally or mentally. Vocal prayer may be spoken or sung. Mental prayer can be either meditation or contemplation. The basic forms of prayer are adoration, contrition, thanksgiving and supplication, abbreviated as A.C.T.S.
We can see that there are two expression of prayer vocally or mentally. The key here is the word "expression". Some say their prayers without zeal and some expressed it more piously as if they feel the presence of God in their midst and they must show reverence and adoration to God. But arriving at this stage of prayer is not easy.
Going back to rosary, the Pope known us Totus Tuus defined praying the rosary as "contemplating the Face of Christ in the eyes of His Mother". If we want to contemplate the Face of Christ, we can do it much better with the help of His Mother who knew Jesus Christ from womb to tomb.
Prayer is like a conversation to God, it is not a monologue. But, it's hard to listen or hear the voice of God, and often times, we never practice listening to God after we say a prayer and after we said our prayer intentions. But, God as a loving Father looks at each one of us like a child growing up. We may not be conversant at first, and we might have difficulty saying things to God, but that's ok.
It's the same thing with the rosary. Our Lady like the loving and merciful Father will stoop down to our level in order to help us know Jesus Christ, She is our Mother too, and like all Mothers they know how to best teach and approach the different temparament of her children.
Praying the rosary even if you were not able to focus or expressed it piously is a good start, and the most important thing is, Satan can no longer claimed that you are his offspring.
We only have two choices in our Christian journey, either we belong to the Woman as her offspring or to the Old Serpent as his offspring. Genesis3:15.
Since one, is starting or making an effort to pray the rosary, it means that they acknowledge Mary the chosen Woman as their Mother, heeding the call of Jesus to Behold His Mother at the Foot of the Cross.
In closing, the benefits of praying the rosary even if one is finding it hard to focus on them, is the powerful words of the late Fr.Gabrielle Amort, the Chief Exorcist of Rome, that says;
"Anyone who goes to Mary and pray the rosary cannot be touch by Satan."