1.why a gaussain noise is added to the channel output in eye diagram??

[bandpass]

gaussain noise is what every engineer from time to time tries to get rid of in telecommunication. isnt it???

in my recent experiment on eye diagram simulation, it was somehow added into, what is the hell purpose??

in addition, what are the changes on eye-diagram? if the original cutoff frequency = 1, the sampling frequency=20 and samples per symbol =10, length of the response=200, roll-off factor =0.15,
now i want to change roll-off factor and length of the response as following figures.

(a) roll-off factor = 0.1, length of the response (N) = 60 & 200
(b) roll-off factor = 0.9, length of the response (N) = 60 & 200

i was told that there is basically no change in eye-diagram for N=60 & 200 in (b), but dont get it, do u all have an idea???