Jyotirlinga is inside you. The twelve Jyotirlingas are in our body itself. Our body has twelve places (where the twelve Jyotirlingas are situated).
Now, in the olden days, the twelve Jyotirlingas were established in different places between Kedarnath to Rameswaram so that people would travel from the North to the South and from the South to the North.
This was a plan that was implemented by our ancestors to make India a unified nation. There was no other way to unite India. How else can you unite India? Every place has a different language, what is the connection between Tamil Nadu and Uttar Pradesh?
They are like two different nations with different cultures, traditions, food, and clothing. Everything is different.
So, people were told to visit Rameshwaram, and then from there they were told to go on a pilgrimage to Kashi and bathe in the holy Ganges. After that they were told to bring back with them the holy water of the Ganges from Kashi to Rameshwaram and offer it to the Jyotirlingam there.
They made it a custom and that is how people from all parts of India got united. Otherwise it would have been divided into twenty four countries like Europe.
Our ancestors have accomplished this very important task of uniting various parts of India. That is why it is said that you must bring water from the river Ganga and offer it in Rameshwaram.