✔ 最佳答案
a tells people that (1) you met your auntie. (2) It was in Mongkok.
b tells people that (1) You went to Mongkok. (2) You met your auntie there.
The difference is (a) does not have the meaning of 'went to Mongkok'. You could be in Mongkok all the time.
If you say "I went to Mong Kok to meet my auntie", it means that you went to Mongkok for the purpose of meeting your auntie. We dont know whether you did or did not meet your auntie.
Hope it helps.