With a wider aperture the sensor can capture more light, helping to avoid blur by enabling a faster shutter speed. It also provides a shallow depth of field, allowing you to blur the background to focus attention on the subject.
Similar to the number of aperture blades, rounded blades affect the way the light gets through to the sensor. Rounded blades, often only found on more expensive lenses, improve the appearance of the out-of-focus areas. This allows you to attain better, softer looking bokeh in your photos.
The aperture controls how much light gets through to the camera’s sensor. More blades is often an indicator of a better quality lens. It also allows you to achieve much nicer looking bokeh when blurring out your background, whereas a lens with less blades will often produce harsher, more polygonal bokeh.