Why optical disk CD/DVD use spiral tracks, as opposed to cocentric tracks used in hard disks and floppy disks?
As the snippet you posted states, spiral is better for continuous audio/video without the need for buffering whereas concentric rings are better for data that isn't necessarily sequential in nature. If you have data like pictures or files or anything else that needs to be read or written in a random manner, then concentric rings would be better because you can have everything indexed at the beginning so that you can jump straight to the ring that contains your specific data.

In summary:
Pros of spiral:


No need for buffering.
Ideal for continuous streams of data


Pros of concentric:  


Indexable
Random access capability


They both have their advantages and when electronics were more expensive, spiral rings were the better engineering tradeoff. Once you can make a cheap buffer, the advantage of spiral ring media is virtually entirely eliminated because a buffer with a concentric ring technology gives the ability to have random access capability while making streaming data equally ideal to use.