Why does Darnay pay to have an escort to Paris?

Darnay pays for an escort to Paris primarily for safety reasons during a tumultuous time in Revolutionary France. The political climate is unstable, and as a French aristocrat, he faces significant risks, including potential imprisonment or execution. The escort provides him with a level of protection against the dangers that come with traveling in such a volatile environment. Additionally, it allows him to navigate the complexities of the situation more securely and discreetly.