I'm sure there's a 'real' name for it. I call it the warning track. It makes it easy to see where the end of the tape is so you can stop rewind/ff before you get to the point that you'll have to thread the tape again. The white makes it possible to locate an area on the tape by looking at the edge too (when it's not on the machine). To make a long story short, it's a marker. On the demo tapes there is a couple of rotations worth of white leader between each track too. That made it easy to jump from one track/artist to another without having to rely on the counter (which, unless a tape/machine has a time code track/reader, the counters aren't very accurate anyway).
BTW the use of some type of leader is important since the first couple of meters of any tape is the most exposed to damage from both handling and environmental/time related gremlins.
