because of - means the reason of somthing, for - means the purpose

They actually mean the same thing. But as the writers say "for" is a coordinating conjunction it causes no dependence between the sentences while "because" causes dependence and the sentences become subordinated.

Because of - The reason for something I can't go on my picnic because of the rain You are explaining the reason you can't go on the picnic For - I haven't really seen it being used much The sentence should be 'I can't see anything because of the fog' I don't think you really use for in this case, I think you just use because of

Using 'for' to indicate the cause of something is a literary form and it is rarely used in normal speech. "Can't see the forest for the trees" is a fixed expression you might encounter that uses this form. In most cases, 'for' is used to mean "on behalf of," (I cleaned up the room for the guests) or specifying the recipient/purpose of an action (I need to borrow a few dollars for lunch. / I called for her.) "Because of" indicates causality. "A because of B" means B caused A to happen, as in the example " I was late this morning because of the heavy traffic".

They can be used top mean the same thing. But 'for' as the reason is an 'old fashioned' formal way of speaking and you will usually only find it in books, poems and formal speech.