I am not sure I got your question right,but if you mean why the phrasal verb is not equal in meaning to the verb it consists partly of ( verb + preposition or verb + adverb or both),it is because the preposition or adverb added to the verb is meant to modify its meaning. However the verb many times doesn't completely change its meaning when adding the preposition or adverb to it as in: ' to break' is to make something separate in two or more pieces, while the phrasal verb 'to break up' means to end a relationship, the meaning here is different ,but somehow has to do with separation of 2 entities ( people here not objects). So in this case the meaning is modified,but not completely different in essence. Another example: 'to get ' is to obtain, to receive, to earn or to buy something, In the case of a phrasal verb such as ' to get away with' here both a preposition and an adverb were added to the verb, the meaning will change completely . 'to get away with ' means do something without being noticed or punished: " He always gets away with cheating on his girlfriend." So those additional prepositions or adverbs are meant to change the meaning, either they change it partly,where the verb keeps its essential meaning with some modification or they change it completely giving the phrasal verb a totally different sense.

Because two words are always different from one word.

phrasal verb means a phrase that consists of a verb plus an adverbial or prepositional particle, esp one the meaning of which cannot be deduced by analysis of the meaning of the constituents . www.thefreedictionary.com here is an example give means to provide someone with something: my sister gave me a pen give up means stop doing something: i gave up smoking two years ago. it means i quit smoking two years ago.

(Mostly) why would you bother to use two if you could actually use one to express the meaning?

pages 364 - 365 might be of interest to you, but I think it's going of the subject abit ^_^ http://books.google.co.uk/books?id=P8Gr3VOK7KoC&lpg=PA365&ots=Qc1eiDTFTt&dq=phrasal%20verbs%20linguistics%20meaning&pg=PA364#v=onepage&q=phrasal%20verbs%20linguistics%20meaning&f=false