Faster aperture = better low light performance.
If you are shooting at ISO3200, f8, 1/100th second with your 18-55mm then you could shoot at:
ISO1600, f5.6, 1/100th
ISO800, f4, 1/100th
ISO400, f2.8, 1/100th
ISO200, f2, 1/100th
Your prime lens will be stopped down a little at f2, very sharp, and considerably better image quality than ISO1600 or ISO3200 with the 18-55
Prime lenses are optimised for one focal length (obviously) while zooms are a compromise throughout their focal range. Obviously the compromise is lessened with more expensive pro zooms, and you pay the $$$ as a result. A cheap 50 is just that though, cheap.
"Bokeh effect", as you put it, is not automatically the preserve of a fast prime. At f5.6, closeup with your 18-55 a distant background will be thrown out of focus. Bokeh is generally considered to be the aesthetic quality of the out of focus areas. Some 50mm primes give nice pleasing bokeh, others less so. What you will get with a 50 f1.4 or f1.8 is the ability to make a subject pop from a background - your Depth of Field will be shallower.
For example, 50mm on a Nikon crop sensor DX camera body with a person 2 metres from you:
at f8, DoF is 51cm
at f5.6, DoF is 36cm
at f4, DoF is 25cm
at f2.8, DoF is 18cm
at f2, DoF is 12cm
at f1.4 Dof is 9cm
Now take another portrait with the person 1 metre from you:
at f8, DoF is 12cm
at f5.6, DoF is 9cm
at f4, DoF is 6cm
at f2.8, DoF is 4cm
at f2, DoF is 3cm
at f1.4 Dof is 2cm
As you can see, in lower light you can shoot better quality images due to the faster aperture. However you have to be more careful with your focusing as the DoF can get pretty shallow. There's nothing worse than a portrait where the eyes have just missed focus unless it's very intentional - you have to be careful with your technique