Building a road for people to walk is necessary as long as the road is not serve for car. It's easy for people get lost in a huge park, and a road can also indicate a direction. It is a way for human beings and animals to live together peaceful. It is a combination of modern society and nature.
Building a road in the middle of a park would greatly disturb the wildlife and could pollute the park. A road would lead to an increase in pollution in the air because of car emissions, increase the noise and disturb/scare off wildlife living near the road, and could increase the pollution of the park overall as people might throw trash out of their cars. Animals curious about the road could get hit by cars passing by. Parks are created to preserve the state of the area as it is, so building a road would only defeat the purpose of having the park at all.
Despite the tourism benefits to building roads in national parks, it is also quite destructive. Also, though it is more beneficial to more people, building roads in national parks would be highly unsatisfactory to those who enjoy peace and remoteness for which national parks are designated. The national parks are also not only for visitation by tourists, but preservation of wildlife which would not be accomplished by road construction.