Its just so the wallet that you are importing it into knows whether to use the compressed or uncompressed public key. It you convert the WIF key back to hex, the compressed private key is identical to the uncompressed one, with the addition of a '01' flag character at the end. I guess its done this way to make it foolproof (the user does not need to specify to the wallet whether to use the compressed or uncompressed public key to generate the address as it is already flagged in the WIF private key string).

Take a look at my script at https://bitcointalk.org/index.php?topic=247178.msg2642261#msg2642261 as it shows the procedure to generate both of the WIF keys from the hex private key quite clearly (just search for WIF). Its simply an 80 prefix. followed by the key value, followed by an 01 suffix (for the compressed key only). To this is appended a double sha256 checksum (just the leading 4 bytes, ie 8 characters in hex), then the whole hex string is base 58 encoded.