Because a laser beam does not diverge, it carries its energy in a very compact form.
To store data in a compact form in a storage medium by taking advantage of known characteristics of the data and the storage medium, in such a way that the original form of the data can be recovered, e.g, to make use of bit or byte locations that would otherwise go unused.
Data in bits can often be recorded into a more compact form for transmission, using a compaction algorithm. A related algorithm can recover the original form ; the entire sequence represents companding.