This is an emphatic interjection, used to indicate dialogical flow and break. In the natural flow of discourse, a brief interjection may be used as a sentence-like enunciation. (At any rate, such expressive use of punctuation and fragmentation is generally not acceptable grammatical composition in formal writing.)

This is an emphatic interjection used to indicate dialogical flow and break. In the natural flow of discourse a brief interjection may be used as a sentence-like enunciation. It can help reproduce or translate dramatic pauses or silences.

At any rate, such expressive use of punctuation and fragmentation is generally not acceptable grammatical composition in formal writing.