Reasons to publish in a "Letters" journal?
While the areas covered by this journal is very broad, the journal focusses on contributions that can be explained in a concise way. They have a strict page limit that is quite low, hence the "letters" in the name.

Other non-letter journals ask for contributions from a more narrow field of research and have a much higher page number limit (if they have one at all). These journals accept papers that provide a substantial benefit to the respective research area, regardless of their length. While the papers submitted to a journal do not officially compete for acceptance (as they would in a conference), all submitted papers need to be competitive in terms of how much they bring the community forward in order to be accepted. Small isolated results often do not fare well in this regard.

A letters journal is hence attractive for contributions that can be explained in a few pages, and for which it makes no sense to embed them in a larger paper (or the author does not want to).