That is strange: the reset button is not (and has never) been connected to the hard disk, i.e. the hard disk is only aware of the reset button been pressed when the BIOS re-initialises the ATA/SATA interface (so a long time after). There isn't any "reset wire" on ATA or SATA connector.
Historically, that has lead to strange bugs - like LILO was able to start after running Windows 3.1 but not from a cold boot (or the opposite), because the hard disk was reconfigured with a different number of heads and sector per track (BIOS only times, no LBA).
For all what I can see, it is exactly the same nowadays.
The reason has always been to give time to the hard disk to finish and write back its cache.