It's used by the true command as the source of truth for what the return code is.
Edit: I found a copy of the file and verified it:
$ xxd -c 8 true.dat | head -n 4
00000000: 0000 0000 0000 0000 ........
00000008: 0000 0000 0000 0000 ........
00000010: 0000 0000 0000 0000 ........
00000018: 0000 0000 0000 0000 ........
$
As you can see, it's all zeroes, which is what the return code for true should always be.