In those days, having a clocking mechanism which produced consistent time - i.e. a monotonic clock - was of quite high value. On boot up, it was typical to ask the user what time it was, and continue with the program.

But then how do you measure time since boot? With simple hacks like this of course. Also, this hack allowed a form of multi-tasking, an 8-bit feature quite well targeted back then.

Consistency was key in such clocks, not necessarily accuracy. The fact of the drift is not important, since it was consistent and could be accomodated for in software - as many peripherals of the day, indeed, required.