There are many reasons for openly carrying a firearm; I will address a few that are most important to me.
1) I am prohibited from carrying a weapon concealed. For some reason, Virginia has seen fit to place the age requirement for a concealed handgun permit at 21 and the possession of a handgun at 18. As I am 20 years old I may not yet carry my gun concealed.
2) In many cases it is more comfortable than carrying concealed. My reason for visiting these parks and forrests is to hike. As a result I usually wear a backpack making it uncomfortable to carry my gun on my hip. A popular option is to carry openly on the leg, a method I have adopted.
3) It is a deterrent for anyone considering doing harm to me, or the people around me. Criminals tend to go for the easy prey. A statistic I heard recently stated that 70% of criminals polled stated they would not attack a person they knew had a gun, or was likely to be carrying one.
4) Some people refuse to apply for a CHP (out of principle) believing they should not need a permit. The State and National constitutions are their permits. I agree with them. A CHP is seen to grant elite firearm owner status, and carrying a firearm concealed an act done only with the permission of the state. A reprehensible idea.