Thanks! I will give You one example and hope You will understand why to use different lenses for different situations. If You use 35mm fixed lens for close portrait You will get very ugly look :) Yours model proportions will be destroyed because of physics and perspective. Better to use 50mm and above (for crop cameras) and 85mm and above (for full frame cameras). This is for portraits.

Landscape photographers uses wide angle lenses, because they want to capture all view and not a part of it (with more mm lens, You will get less of that landscape in Your photo) :)