The short answer is this - and more can be found in this particular article on our website: Yom Kippur/Atonement.
Since Y'shua was our Final SIN Sacrifice/Offering, why do we have to bother with Yom Kippur? Well, Y'shua died to cover our sins so we could enter the Kingdom in spite of our sin nature. Thanks to him, our debt is covered - but the thing is, we still inadvertently sin and Yom Kippur is the ELOHIM-commanded day, a permanent regulation, to seek forgiveness for those sins!
On Yom Kippur, we are to spend the whole day remembering where and how we have wronged our Creator, and to seek His acceptance of our remorse and ask Him to forgive our errors....