Horses were the chief means of transportation during the period of Gurus. Guru Hargobind travelled a lot hence necessity of good horses. Also he had to fight the Mughals specially those who troubled the masses. in battles with Mughals the horses were of immense use for mobility. Need for Weapons also arose during these battles.
'You have to go back to the history to understand the needs of the time. The first five Gurus had not used horses and weapons. The need of these arose only once the government of the time lost the real sense of justice system. They used sword unabashedly to spread their religion and to have their power and they had no human values as such. This was on the increase. The heat was felt by the 6th Guru after the martyrdom of 5th Guru. Chivalry was a least choice but offensive defence was the urgent necessity since Mughals did not understand the land of reason and human aspects. They only understood the language of weapon and force. To stop them from this having weapons was an urgent necessity felt by the 6th Guru. For battles then horse was essential for mobility and the two were a combined force hence used as such.