So it's for the proper 'alignment' of the partitions.
Thanks for the link, it was very helpful.
If you haven't done anything with the disk yet it may be worth converting it to a GPT type disk -- you won't have to use extended partitions (it allows 128 partitions as standard) and a backup copy of the partition table is stored at the end of the disk in case the main table is damaged. It will also make it easier to convert to EFI-mode booting at a later date (EFI-mode booting with MBR disks is firmware-dependent and may not work at all).
Oh, I don't think I need GPT yet for my hardware.