Why are chromosomes important to heredity?
Chromosomes are the carriers of genetic information.
Heredity, in terms of genomics, means the transfer of genetic data/information from parents to offspring. This 'data' is encoded in the DNA in terms of nucleotide sequences.
During cell division, each DNA helix in the cell coils up to form a chromosome which then acts as a package carrying genetic information from the parent cell to the daughter cell.
However, if you meant "Why is the formation of chromosomes important to heredity?", the physical property of coiling and compacting makes the DNA resistant to degradation by Dnases or destruction by breaking. Also, the condensed structure makes it easier for complete genetic data to be transferred from the parent cell to daughter cells in good condition or without loss.