Why is the rough ER necessary to make extracellular proteins?
Extracellular proteins are proteins that function outside the cell, meaning they are made in cells (transcribed from ribosomes) and then are transported to the plasma membrane to be delivered to the extracellular space. The proteins that need to be secreted aren't secreted as free proteins, rather they are packaged into vesicles. Vesicles are membrane-enclosed structures. When ribosomes are translating mRNA and come across a specific sequence called a signal peptide (wiki), the ribosome goes to the ER and delivers the peptide into the ER. From here, the peptide gets packaged into a vesicle-- first a membrane budding off from the ER. 
 source(NCBI)

This is also how secretory proteins get into the Golgi, by vesicle trafficking. From the golgi, proteins are packaged into vesicles. It is a hotly debated topic whether proteins are sorted into specific vesicles or proteins within vesicles (that do not belong in that vesicle) are sorted away (and degraded by the lysosome).