Sounds too good to be true. What are the downsides? You say that it reads a location that was calculated, but then also checks the crc32 and if it doesn't match

Sounds too good to be true. What are the downsides? You say that it reads a location that was calculated, but then also checks the crc32 and if it doesn't match it will move to the next calculated position. Why is reading the crc32 needed? Why doesn't it immediately go to the next position?