When we speak about an object, we use (c'est): - it's a table -> c'est une table - it's logical -> c'est logique - it is broken -> c'est cassé But when we speak about a human being, we use (il *masculin* /elle *féminin*) - she is happy -> elle est heureuse - he can't go -> il ne peut pas aller - I was with her -> j'étais avec elle - I was with him -> j'étais avec lui (here we can't use (il) so we used (lui))

Sorry, on further thought it seems my answer was not exactly addressing your question, since both 'il' and 'ce' can function like English 'it'. So it is not just like the difference between this/that and it. If your question is important enough, I suggest you ask again giving more context. As it stands, it seems it is not clear what usages you are referring to.

I would say for the same reason you use "it" instead of "this" or "that" in English. You wouldn't say "That was at that point I realized ...". You have to say "it" - "It was at that point that I realized ...". You can't say "That(this) is wrong to lie.", but must say "It is "It is wrong to lie.". "it" and "this/that" has entirely different usage. It's no different in French.