This mission introduces you to Roman's taxi business and the concept of driving passengers. It's a relatively simple mission, but it teaches you the basics of taxi work, which can be a source of income later on.
Roman, ever the entrepreneur, needs Niko's help with his taxi business. This mission serves as a practical tutorial for driving passengers around Liberty City, a core mechanic that can be utilized for side income. It's a break from the violence, allowing you to focus on driving skills and navigation.