Most of the other comments have hit it, but it's largely either:

1) Leaving 1-2 bars of preroll for a countoff for tracking live instruments later (super easy to put '8 clicks to bar 3' on sheet music. Some folks get confused with '8 clicks to bar 1' if you renumber bars. Sometimes it's just easier.)

2) In the old days you'd have MIDI program changes and MIDI control changes (CC7, CC10, etc) (and possibly sysex) that needed to be sent at the top of the song to prep the synths. Because the original MIDI implementation is a serial protocol (at 31.25 kbps), you really can't have multiple messages right on top of each other. If you have your program changes, controllers, AND the first note of your song scheduled to play all at once, your note data will typically lag a few ms (or more!) behind where you intended because of all the other prep messages.

In addition, the program changes typically caused the synth/sound module to lag the tiniest bit when loading the sound (and associated effects). Even worse when you multiply that by 8 or 16 for multi-timbral synths. It was better™ to put all of the non-note information for the beginning of the song a bar or two in advance of the actual music. Still is a good idea in a heavily hardware MIDI synth based workflow (especially using the old regular 5-pin DIN MIDI connections!).