In the Hebrew Bible, specifically Leviticus, why was Yom Kippur commanded to be celebrated when...?

...the Israelites had to offer sin and guilt offerings throughout the year anyway? I had the impression you had to offer a sin/guilt offering whenever you broke part of the Mosaic law, or "remain unclean until the evening and then wash yourself with water" if you wanted to be ritually purified. Did I misunderstand something? I'm learning about Judaism right now and am reading the Hebrew Bible. Leviticus is turning out to be very difficult!