The term index is used to define an ordered list of files which are meeting certain criteria. The simplest and most common index is a group of files in a directory, sorted by name.
Thus, such indexing used in NTFS filesystem (storing copies of some attributes) can lead to great performance improvement since the number of disk read requests is decreased. For a regular operation, NTFS filesystem requires both Master File Table records and indexes. But it should be noted that to restore data from the NTFS file system, indexes are not required for MFT entries alone are sufficient. This is because exactly the same data as in index is contained in MFT records themselves. So it is not required to take into account indexes when extracting data from the NTFS filesystem.
But in case of chkdsk tool, it has to make sure that all the indexes are in a consistent state, otherwise NTFS filesystem is not able to work in a normal (fast) way. It is interesting that if chkdsk tool works on the disk which is copying data, then the tool may show false errors. This is due to MFT and indexes updates may be not synchronized.