Basically this: they are added as I see fit, to be disambiguations (explicitly these Maori instruments, as I felt they where confusable with eachother somewhat.)
The "Description" section is a remnant of when instruments where just text-entries, nested in a tree, not entities with mbids in their own right. Another remnant is the "child of" relationship which is being phased out (I'm working on this) as well as things like INST-330
We discussed this in #mb, solutions to this include moving description text to annotations (and adding annotations to instruments if they do not have them yet), removing description text from searchview and just dropping descriptions altogether (because it mostly duplicates Wikipedia text without a credit)
Generally I try to make the disambiguations as short but descriptive as possible, and I reword the descriptions, explicitly, to make them not just be copypasta from wikipedia.
I've just started working on instrument requests, and I've just started adding and working on these instruments. I will generally add the text I feel is sufficient - as well as the links and rels possible, to identify and connect new (and old) instruments. It's kind of a huge endeavour (as many as 200 open instrument requests!) in the future I might go back on these I've added and redefine or remove text (if at all necessary then, see previous paragraphs about solutions) but I will not do so, now.
FTR, there are also plenty of instruments without a disambiguation or a description (I'm working on that, too)