I have just started studying fluid mechanics and on the very first page two assumptions are made about liquids while I understood the first one but am not able to understand the Second one: 

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> Parts of the liquids in contact do not exert any tangential force on each other. the force by any part of the liquid on the other part is perpendicular to the surface of contact. thus there is no friction between adjacent layers of liquids. 
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why are such weird assumptions being made here? what do they want to signify? Like if there is friction between adjacent layers of liquids so what? Why assume there is no friction?