There is a man of the roof
A man is on the roof
both sentence are giving information.
my questions is why we use introductory subject?
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In this use, there" is only ever an adverb. There is a common misconception that "there" can be a pronoun. There means "at that place." (Here means "at this place" and "where" means "at which place").
We know that there is an adverb because there cannot be the subject of the sentence:
There is a dog in the garden (A dog is in the garden.)
There are two dogs in the garden. (Two dogs are in the garden.)
Because, in English, the verb's form is dictated by its subject, the subject cannot be "there", the subject must be "dog" and "dogs" respectively.
OED description and comment:
- Used unemphatically to introduce a sentence or clause in which, for the sake of emphasis or preparing the hearer, the verb comes before its subject, as there comes a time when, etc., there was heard a rumbling noise. In interrogative sentences there comes between the verb and subject, as Breathes there the man, etc.?, or follows the first word of a compound verb, as Does there breathe a man?, Shall there be any notice taken of it? The same order was formerly observed after an introductory adv. or clause, as Then came there a voice, Soon shall there arise a prophet. Grammatically, there is no difference between There comes the train! and There comes a time when, etc.; but, while in the former there is demonstrative and stressed, in the latter it has been reduced to a mere anticipative element occupying the place of the subject which comes later. Preceding or following a main verb, or following any verb, there, thus used, is stressless (proclitic or enclitic: e.g. there-ˈcame, ˈbreathes-there, ˈis-there, ˈwill-there), but preceding be or an auxiliary, there has a slight stress, and the verb is enclitic (e.g. ˈthere-is, ˈthere-was, ˈthere-will).
The inversion of subject and verb is relatively common when a sentence is fronted by an adjunct:
Only after the war was over did he go home.
Dearly do I love you.
Many are the times that I thought of you. (Many are the times -> The times exist numerously.)
Thus, there may be
(i) weakly demonstrative and merely indicates the existence of an item -> “There is a dog in the garden.” Or “There are lions in Africa” This is because the only way that the verb “to be” can take an adverb is in its meaning of “to exist” (or "have an existence.")
(ii) clearly demonstrative when used with a non-stative/dynamic verb: "Look there!"
It might seem that there can be a noun meaning “that place [yonder]” e.g. “He left there last night.” This occurs only because the other forms of thence and thither [size=2]which indicate motion[/size] have been all but lost and “he left there last night” = He left from that place last night, in which “from that place” is adverbial and likewise “thence “to that place””.
 There, here, and where also may substitute for thence, thither and hence, hither, and whence, whither with the appropriate change of preposition.
We call "there" existential or introductory. One can express himself by any other plausible means. But that doesn't mean we have no need of Introductory THERE. Often times it is essential and lends beauty and elegance to your writing.
Don't begin, " In ancient times....."
The sentence would be lackadaisical.