Why do you think the poet had to know this long poem by heart?

In Anglo-Saxon times very few people could read and write, and there were not many books. This was long before the days of printing. Books had to be copied out by hand onto pages made from dried calf-skin (called vellum). This was done by monks, because they knew how to read and write. Then they sewed the pages together. Often the monks decorated the first letter of each page very beautifully like this:

So poetry was not often read in a book. People listened to it all together, as the Danes listen to the poem of Finn the Frisian in Heorot. The poem Beowulf was probably first heard at a feast like this.

An Anglo-Saxon poet was called a scop. The scop knew long poems off by heart. These would tell of battles or the deeds of heroes or gods. While telling the story, the scop would play on a harp or lyre. Parts of an instrument like this were found in the ship-grave at Sutton Hoo. Here is how it might have looked:

This is an Anglo-Saxon picture of King David (from the Bible) with his musicians. He is playing an instrument like the Lyre you see above.