1. Time is an artificial concept of measurement.
2. The purpose of time in our existence is to know when our ramen is ready.
3 &4. Going to a future too far away could be a high risk, but, going back to a past period could be more manageable if one knows the history well. However, using time to one's favor might disrupt the timeline and fall into chaos. Also, the choice of time travel is affected whether or not if two of the same person can exist in one timeline. Aside from the sci-fi mumbojumbo, I honestly don't know how I will make usage of time travel. Going back to the past could make a person relive all the previous pains and going to the future could result in seeing how the world have already failed by then. Simply put, the past is not a utopia and the future is too uncertain. In a more hopeful perspective, if one believes the possibility of the humans, going to the future can be a positive gamble if one thinks that the world would overcome its flaws by then. Actually, after looking at how many minutes I used to write this comment, I would go back in time before all this. Aditionally, if one is a big cultural fan of music and movies, I can recommend to go to the 80's, as it was an excellent period where many masterpieces, legendary musicians, and the American dream were still alive; while the fear of a nuclear war was real.
5. Time is not an entity and cannot say if it exists or not exist. However, as long as there is any form of life in this universe, the sense of time passing can exist.