flatMap is always so unituitive (not a word) for me for some reason. I messed with it a bunch in C#'s LINQ and RxJS and I already forgot the purpose. Not sure what it is that makes it so unnatural for me.
However, generally you don't want to operate on a list of lists and are trying to process each value one by one -- the nesting doesn't add anything. In this case, we use flatMap, which "flattens" or concatenates the interior lists so we can operate on them like it's just a big stream of values.
This is also the case for another type like `Optional`, which represents either a value `T` or the absence of a value. An optional can be "mapped" so that a function is applied only if there is a value `T` present. flatMap works the same way here, where if you want to call another method that also produces an `Optional`, flatMap will "unwrap" the optional since you never really want to work with the type `Optional<Optional<T>>`.
'map' as a function name isn't great either, since we have the same name for a data structure. What it has in its favor is being short and traditional.
JS flatMap = C# SelectMany