Christians, Atheists and Catholics: Why is "guarding" the Blessed Sacrament during Exposition so important?

I recently passed a chapel in a Roman Catholic Church during Exposition and the older person who was with me became frenetic, when she thought that no one was guarding the monstrance with the Blessed Sacrament in it. I don't get it. If someone were to come in the chapel with designs on being sacrilegious towards the Blessed Sacrament, it would not be as if they could manipulate "God" in the form of bread. I don't see God being "mistreated" or "misused." I believe it would counter his ability to be all-powerful. What do YOU think?