This trial was highly sensationalized and also was under the control of what was then known as the "radical Republicans".
Johnson who was elevated to the Presidency by the assassination of Abraham Lincoln was not liked for several reasons, perhaps none more so than the fact that he was first a Democrat, and secondly he was willing to accept the easy entrance back into the Union most of the southern states. Lastly, he was very vocal in his criticism of Congress and very willing to veto anything he felt was not useful to the country.
The Congress was waiting to put in their "lackey" Grant, in the Presidential chair. Of course that would happen in 1869 and Grant would be as bad a President as could be expected. Of course some really bizarre historians these days think he was good? Well he was as good as Harding, but that is another story.
The radicals in Congress wanted to punish the south and also push some of their rather radical agendas on areas that they hated.
Johnson was not a great President it is true, but he was for all intents a victim of the radicals, who wanted to remove him.
The trial would have changed the power of the President if he was found guilty of the charges. The article here talks about the trial as it was going on and the question of if he would be found guilty and removed from office.
This series of pages deals with the news as it was known then. It makes fascinating reading as many histories will leave out little details here and there that made the trial interesting. Here you can read it all. This is indeed history when it was news.