Why does Northrop travel with Brown and Hamilton?

Brown and Hamilton paid Northrup to travel with them and entertain them during their journey by playing the violin.

They also remarked that they had found much difficulty in procuring music for their entertainments, and that if I would accompany them as far as New-York, they would give me one dollar for each day's services, and three dollars in addition for every night I played at their performances, besides sufficient to pay the expenses of my return from New-York to Saratoga.