It saves you needing a separate index variable. If you're doing something like LZSS decompression it makes the relative window offsets natural (absolute). Not sure I'd recommend coding like that today, unless ptr was a smart pointer that had bounds checking, but for the time period it was a perfectly fine technique.

But the destructive nature of ++wptr or whatever can be to its advantage, in that you never need to update any numbers when things change. The data is consumed/produced in the order specified, a property that is maintained even after new operations are inserted or existing operations are reordered.

(Whether you want this or not, who can say! - though I claim you typically do.)