Chinese words have entered Mongolian steadily for centuries, by way of trade, administration and, in Inner Mongolia, daily coexistence. Writing them in the traditional alphabet is not straightforward. Chinese syllables are built to a pattern Mongolian does not share, with initials, finals and tones, and a Mongolian alphabet designed for a language with vowel harmony and a modest consonant inventory cannot represent them without either loss or extension. Several of the ten added letters were supplied for exactly this, including this velar, the dental affricate, the voiced sibilant and two letters that stand for whole syllables rather than for single sounds. That last device is worth noticing, because a syllable letter in an alphabet is an anomaly: it means the tradition judged the sound impossible to build out of the letters available and reached for a single sign instead.
The pressure was practical rather than literary. Names, offices, goods and places had to be written down correctly in documents that would be read by people who needed to find them again, and an alphabet that could not do that would have been extended sooner or later whatever anybody thought about it. The Buddhist literature exerted the same pressure from a different direction and at a much greater scale, which is why the two efforts, the letters for Chinese and the apparatus for Sanskrit and Tibetan, sit side by side at the end of the same alphabet.