Ok, there are number of things that I wanted to say about going back to time. Yes, there will be cause and effect, by doing so and you will never know what will happen if the history is changed.
However, there
will be real time machine in 30 years from now. Scientists had been doing much research about it. (Those ebay one is fake.. trust me) The time machine might not be in use for the public but the military. Imagine if everyone used the time machine, what will happen to the world? Unless those professional people in the military are well-trained enough to go back to recover lost info or document.
I heard people saying about going back to time to 'save' the world? (tsunami for example) Well, if you really gone back to time and reported about what will going to happen before the tsunami strike, will anyone from that 'past' believe you? ok, let say Im from the future and im now back in time. I'm telling you all that there will be earthquake in singapore 1 hr later from now (touch wood). Will everybody evacute singapore just because I said that? (Ok, not really evacute but at least went to so-call safety shelter.) And if i told fire station about it, will they sound the alarm just because i said that? (1hr is a short time though)
In your shoes, you might think that if people dont believe me, they will die. So you try to make your stand to the government. Ok, the earthquake did happen (touch wood again) but after that what will you think that are going to happen? Yes, you save alot of life but you expose yourself as a time traveler. You will end up in the lab being research by the scientist

and they will force you to tell the truth about the machine.
For your information: There's infinite worldline as in when you travel back in time, you might land up in different worldline which is slightly different from current worldline(now). Furthermore, how are you going to face 'yourself'? Since any “you” on another worldline would not be a mirror image, you get a slightly distorted feeling while seeing yourself, let alone the concept of speaking to yourself. (will not talk much about this as it's kinda off-topic and no one might be interested anyway

)
Thank You