Scientifically just taking a walk can reduce chances of heart related problems, brings up the heart rate, lowers blood pressure and strengthens the heart, but i don`t think that is the reason why people take long walks. That is more of a hidden advantage, now these are the reason why i personally take long walks.
The first and obvious reason is, i do not have a car. Yes i know most of you guys have cars and you use them to go everywhere, to and from work, to the grocery shops and i am i sure if i had one too, i would not take long walks that often.
The second reason i take long walks is to calm myself down. I have a brother and sometimes we get into arguments but if the situation keeps getting out of hand, i always take a walk, 5 to 8 km, just walking, rethinking what really started the argument, was i wrong the wrong or not and what can i do to resolve the situation by the time i get back home. To further calm myself down, i always listen to music while i take these long walks. Music is also helpful when it comes to controlling our emotions.
I also take long walks to clear my mind and regain focus. I usually do this at school between lectures. You can have a 2-3 hrs lecture and only an hour of break time before the next lecture. Taking a walk, a short one around campus in this case can help you to brainstorm the previous lecture as well as clearing your mind and making it ready so that you will be more focused in the next one. And just taking walks in general helps you clear your mind.
Lastly this is more of a personal preference for me, when ever i want to plan something for the first time, i take a really long walk because first i have to clear my mind of unnecessary thoughts and focus on thinking about the plan i will be constructing, so taking long walks really helps me to effectively make plans and better decisions for my life. After all, when i am taking a walk, it is just me and my thoughts.
Feel free to share reasons why you take long walks in the comment section.