Why we use Triangle wave in mobile jammer?

Assume you mean the control voltage for a VCO?

The shape and rate of this voltage will define the output spectrum of your oscillator. Often the goal is equal spaced spectral energy and this will be acheived by using saw tooth shaped sweep, with shortest possible retrace. Using a triangle shaped control voltage will give greatly varable amplitude on your spectrum.

The 555 might do OK, but not for critical appliccations.

Noise is often a better/simpler way :)