It is necessary. We can, of course, say simply 'I saw it'. Or, if you really want to emphasise the fact that this was something that you witnessed yourself, you would use the set phrase 'to see something with one's own eyes' : 'I saw it with my own eyes' We wouldn't say 'I saw with my eyes'. This is not a set phrase - it would be a straightforward statement. However, as a simple statement it would be a strange thing to say, because there is obviously no other way to see something than with your eyes.