Why is it important to find out who the speaker/author is in a poem?

This is a
good question. There are two issues here - the author and the speaker of the poem. Let me deal
with the author first and then the speaker.

It is important to know something
about the author to appreciate a literary work in greater detail. If you know something about
the culture of the author, then you might gain some cultural insights. For example, to know that
Homer's Iliad was written in a culture that prizes courage, cunning, and
glory is very helpful. The more you know of the historical and cultural context, the more you
will be able to understand a poem.

Second, the speak of the poem does not
have to be the author's voice. The author can uses different perspective. In literary circles,
this is called focalization. So, an observant reader will always seek to determine from whose
perspective is the story being told.