To protect us from the harmful ultraviolet rays of the sun
Most of the ozone in the atmosphere is in the stratosphere of the atmosphere, with about 8% in the lower troposphere. As mentioned there, the ozone is formed due to photo reaction. Information regarding a current state of stratospheric ozone, check the environment Canada Internet pages.
The ozone level is measured in Dobson Unit (DU), named after G.M.B. Dobson, who investigated the ozone between 1920 and 1960. One Dobson Unit (DU) is defined to be 0.01 mm thickness of ozone at STP when all the ozone in the air column above an area is collected and spread over the entire area. Thus, 100 DU is 1 mm thick. The animated picture shown here is from this link, which shows the ozone hole in the artic region. Often, the observed thickness is 300 DU.