While the iliad does relate to actual events, it should not be taken at face value. Regarding your question, destroying the ships would be very useful in crippling the logistics of the besieging force. An army needs to be fed, reinforcements must be brought in, wounded and dead evacuated. Without the lifeline offered by a navy, an invading army is forced to live off the land, which is hard to do if all you have is the beach next to a fortified city. Without the ships to bring all the resources necessary to keep an army in the field, the siege would have had to be abandoned.