Bobby Fischer had 'invented' a way of incrementing some time duration with every chess move(s) played. This was quite new in that era. But was that so unique that a patent was granted for it?
Here are those patent details of "Digital Chess clock".
was that so unique that a patent was granted for it?
According to patent law, yes. There was no device in common use that did that and no previous similar patent had been filed. Hence his idea was patentable. You can't patent fire because it has been around in everyday use long before patents. Similar for steam engines. Patents only last for a relatively short period to allow you time to put your idea into production.
What was so special about the digital clock is something else, or rather several things.
The first is fairness and a better game. Every time you make a move you get a bit more time to make your next move. You only lose on time if you take too much time on your last move.
It is also much better for the equipment. Players no longer feel the need to hit the clock as quickly as possible to save that fraction of a second which might allow them to make an additional move.
It also takes the arbiters out of deciding the game when one player claims a draw with less than 2 minutes left on the clock. In the absence of cheating or unsportsmanlike behaviour it is always best and fairest if the game is devided by the play of the two players.