Random signls and clocktime in Simulink

I have a problem in understanding a simple matlab property. If you have this simple model with the clock, a matlab function and a scope. Why does the fuction is just run every 0.2 seconds in the simulation time? Same when you use 100 as Stop time. Then the function will be run every 2 seconds. In between the random values generated by the funtion, Simulink connects the values linearzed. Next question is why is simulink generating alwys the same "random"signal? I want to implement a random noise signal in a bigger model, but i think i did not understand how it works.

function y = fcn(u)
y  =rand;
end