新概念二册96课为什么整篇用一般现在时，而且有的句子是用过去完成时与一般现在时来表示动作前后关系？

新概念二册96课为什么整篇用一般现在时，而且有的句子是用过去完成时与一般现在时来表示动作前后关系？
A Festival for the Dead is held once a year in Japan. This festival is a cheerful occasion, for on this day, the dead are said to return to their homes and they are welcomed by the living. As they are expected to be hungry after their long journey, food is laid out for them. Specially-made lanterns are hung outside each house to help the dead to find their way. All night long, people dance and sing. In the early morning, the food that had been laid out for the dead is thrown into a river or into the sea as it is considered unlucky for anyone living to eat it. In towns that are near the sea, the tiny lanterns which had been hung in the streets the night before, are placed into the water when the festival is over. Thousands of lanterns slowly drift out to sea guiding the dead on their return journey to the other world. This is a moving spectacle, for crowds of people stand on the shore watching the lanterns drifting away until they can be seen no more.