Darnay needs to hire an escort because he cannot pass through Paris without constantly being stopped for his papers.
When Darnay gets to France, he is told that he can’t travel without an escort because he is an aristocrat and an emigrant. Basically, they know who he is and that he went to England. He is the Marquis St. Evremonde, and therefore hated throughout the new order.
“Silence!” growled a red-cap, striking at the coverlet with the butt-end of his musket. “Peace, aristocrat!”
“It is as the good patriot says,” observed the timid functionary. “You are an aristocrat, and must have an escort—and must pay for it.”
“I have no choice,” said Charles Darnay. (Book 3, Chapter 1)
The men laugh at him. They are wearing red caps, and tell him he is going to be judged in Paris. This is the beginning of the real danger for Darnay, because just by coming back he risks getting the guillotine. Darnay would risk anything for a friend though, and goes on.
Charles Darnay had to hire the escort because:
The environment was not safe for him given he was considered an emigrant and an aristocrat even though he tried to state he was a citizen. "It is as the good patriot says," observed the timid functionary. "You are an aristocrat, and must have an escort—and must pay for it."
He would not arrive in Paris in time because of the endless check points where he would have to identify himself throughout his journey and where the patriots would have turned him back or even held him in confinement given he was not in the good books of the new order.
“This universal watchfulness not only stopped him on the highway twenty times in a stage, but retarded his progress twenty times in a day, by riding after him and taking him back, riding before him and stopping him by anticipation, riding with him and keeping him in charge.”
He needed the escort to help him arrive in Paris safely because he had to clear his name.
Apart from the guards and the checkpoints, Darnay needed the escorts to protect him from the villagers who considered him a traitor and would harm him if they had a chance.