There is a ten-day period between the creation of Beowulf's pyre and the final construction of his barrow. Practically speaking, this waiting period is due to the construction time. How can this wait period be interpreted on a metaphorical level?
Because of Beowulf's sudden death, the Geats needed time to mourn.
In a time of crisis, the Geats wished for Beowulf's protective spirit to remain among them as long as possible.
Having died in battle, Beowulf's soul is restless and needs time to wind down.
Upset that Beowulf had abandoned them, the Geats needed time forgive their king.
This space of time allowed Beowulf soul to go wherever it was destined to go.