Concerning your question about the epic poem Beowulf, epic heroes from Achilles to Beowulf are obsessed with being remembered. They gain immortality, in a sense, by being remembered for their deeds.
Beowulf wants to be remembered. The practical benefit to the tower might be to guide ships like a lighthouse, but really Beowulf wants to be remembered--he wants immortality.
As Beowulf says:
The brave Geats build me a tomb,
When the funeral flames have burned me, and build it
Here, at the water's edge, high
On this spit of land, so sailors can see
This tower, and remember my name, and call it
Beowulf's tower, and boats in the darkness
And mist, crossing the sea, will know it. (823-830)