Compound sentences are syntactic structures which part two and more simple sentences are. There are compound and complex sentences.
To understand why compound sentences are necessary, it is necessary to define at first, than they differ from simplein offers. From the point of view of the building the compound sentences represent more branched structure including two or more predicative bases.
But It should be noted that to a compound sentence it is possible to give as an analog two simple without prejudice to sense. For example:
1) ""There came the spring, from the distant countries birds"".2 returned) ""There came the spring. From the distant countries, birds returned"".
And in a case with a complex sentence of similar replacement it is impossible to make. For example:"" I left the room which was crowded with people"". If to try to divide this offer, then its first part will lose the initial meaning, and the second will be informatively insufficient and incapable to function as separate syntactic unit. It is connected with a semantic system of various types of a compound sentence. If the compound type is characterized by equal relations of predicative bases, then cause and effect, space-time, comparative, adversative and other types of the relations are the cornerstone of a complex sentence. And these relations can be implemented only in structure of a compound sentence. It is important to note that the compound sentence, unlike simple, often includes synthesis of the various semantic relations. Offers with adversative and comparative value belong to them: ""The sister already works, and the brother still idles"". Or a design with adversative and compensatory value: ""Art - a burden on shoulders, but as we, poets, appreciate life in fleeting trifles!"" (A. Blok). Thus, compound sentences first of all are necessary for display of various semantic relations between components. Besides, these offers allow to make the speech more informative and expressive. Function of style differentiation also plays an important role. Compound sentences are characteristic of book styles and a written language.