DES has a 64-bit key size, but only 56 of those are used during encryption. The other 8 are "parity bits".
What was the intended purpose of the party bits, and why are they no longer used in modern ciphers?
They are there to check if the key was indeed correctly retrieved. It could for instance be that the key is a result of key decryption or key agreement. In that case, or simply during transmission, wrong keys are used. According to NIST FIPS 46-3:
The 8 error detecting bits..."
Or even better, Wikipedia states ANSI INCITS 92-1981), section 3.5:
One bit in each 8-bit byte of the KEY may be utilized for error detection in key generation, distribution, and storage. Bits 8, 16,..., 64 are for use in ensuring that each byte is of odd parity.
So you see, they're there to protect against errors reinstating/recreating the key.
Nowadays transmission errors are usually taken care of at the transport layer. As e.g. TCP/IP (and most other network protocols) already deliver a reliable transport mechanism the need for the parity bits has been strongly reduced. Things like parity checking are not seen as part of a cipher definition. Nowadays it is seen as an unnecessary nuisance, complexity where it isn't required.
In the unlikely event that you'd ever need to perform parity or CRC checking it is of course easy to add the necessary bits; as long as you strip them away again before using the key.
Parity of DES key bytes was introduced on request of US authorities during the design of DES in the late 1970s:
A strong argument about the reality of the second motivation is on page marked 232 of a partially declassified book edited by the NSA: American Cryptology during the Cold War, 1945-1989, in the less redacted version found in page 5 of the PDF in this zip archive obtained by John Young under FOIA request 60251 of late 2009:
NSA worked closely with IBM to strengthen the algorithm against all except brute force attacks and to strengthen substitution tables, called S-boxes. Conversely, NSA tried to convince IBM to reduce the length of the key from 64 to 48 bits. Ultimately, they compromised on a 56-bit key.
Here is the union of said two differently redacted versions of the bottom of that page