Why need to use " ' " in some English words?
E.g.
1. Betty and Lingling are standing in front of Tian'anmen Square. [Tian'anmen Square]
2.Yes, there is a big bookshop over there, just along Xi Chang'an Jie, on the right, opposite the bank. [Xi Chang'an Jie]
3. Excuse me, can you tell me the way to Wang fujing Dajie? [Wang fujing Dajie]Yes it's a Chinese place too but it don't need " ' ", so I'm not sure if it's a problem with Pinyin.

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In the US, it is most commonly written "Tiananmen" without any marks. It is most accurately written"Tiān’ānmén".

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The apostrophe here has nothing to do with English - it is to help the pinyin IME - it is NOT necessary in fact for tiananmen since there is no 'tia' in Chinese, and the IME knows it must be tian.  However when you write xian 先/现 or xi'an 西安 the apostrophe is a hint for the IME.
In written English sometimes we use a dash '-' similarly to help the eye interpret the word, as in 'co-operate' co-ordinate, co-opt...

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I think it's just from transliterating the Chinese characters. I can see that it's supposed to show how to read the syllables, ie. Chang an, not Chan gan; and Tian an men, not Tia nan men. However, it's not really a standard practice in English to write words in this way, and not really how we use the apostrophe ( ' ) anyway.