U+3164 HANGUL FILLER is one of the stupidest choices made by character sets. Hangul is noted for its algorithmic construction and Hangul charsets should ideally be following that. Unfortunately, the predominant method for multibyte encoding was ISO 2022 and EUC and both required a rather small repetoire of 94 × 94 = 8,836 characters [1] which are much less than required 19 × 21 × 28 = 11,172 modern syllables.
The initial KS X 1001 charset, therefore, only contained 2,350 frequent syllables (plus 4,888 Chinese characters with some duplicates, themselves becoming another Unicode headache). Notwithstanding the fact that remaining syllables are NOT supported, this resulted in a significant complexity burden for every Hangul-supporting software and there were confusion and contention between KS X 1001 and less interoperable "compositional" (johab) encodings before Unicode. The standardization committee has later acknowledged the charset's shortcoming, but only by adding four-letter (thus eight-byte) ad-hoc combinations for all remaining syllables! The Hangul filler is a designator for such combinations, e.g. <fliler>ㄱㅏ<filler>
denotes 가
and <filler>ㅂㅞㄺ
denotes 뷁
(not in KS X 1001 per se).
Hangul filler was too late in the scene that it had virtually no support from software industry. Sadly, the filler was there and Unicode had to accept it; technically it can be used to designate a letter (even though Unicode does not support the combinations) so the filler itself should be considered as a letter as well. What, the, hell.
[1] It is technically possible to use 94 × 94 × 94 = 830,584 characters with three-byte encoding, but as far as I know there is no known example of such charset designed (thus no real support too).