In the book Mrs. Frisby and the Rats of NIMH, what was the purpose of the "annual light bulb harvest?"

The purpose of the annual light bulb harvest is to provide
the rats with light for their living quarters. As a result of the experiment that was
done on them at NIMH, the rats are highly intelligent, and have, among other things,
been able unlock the secret of electricity. The entryway to their underground home is
kept dark by necessity, to avoid detection, but once one gets to the main living area,
which is three feet under the ground, everything is lit with tiny lightbulbs. These
lightbulbs have been placed in strategic recesses along the walls and covered with
squares of colored glass, making them look like "stained-glass windows in sunlight." To
secure their light bulbs, the rats go out in the neighborhood and take them from
people's decorated trees at Christmas time. They take only a few bulbs from each tree
they come across, again to escape detection; to get enough bulbs to illuminate their
living quarters to their satisfaction, they had had to go "quite far up the road" for
two consecutive Christmases. Justin calls the process "the annual light bulb harvest,"
and describes the experience dreamily "like picking fruit" (Chapter 11 - "In the
Library")