It's for sector-alignment on the newer Advanced Format drives with 4K sectors, and for the future. It's apparently mentioned in a now-withdrawn Microsoft Knowledge Base article, captured and quoted on Wikipedia:
In earlier versions of Windows, the default starting offset for the
first partition on a hard disk drive was sector 0x3F. Because this
starting offset was an odd number, it could cause performance issues
on large-sector drives because of misalignment between the partition
and the physical sectors. In Windows Vista, the default starting
offset will generally be sector 0x800.
If in the future we go to 16K or even 1MB sectors, you can image an old disk onto a new one, and the partition will still be aligned.
The half-MB blocks on SSDs that Robert mentioned also makes sense. If you're going to realign something, pick a nice round number.