What will happen if i remove seeds from a chilli pepper before cooking?
You remove the seeds for texture. You remove the white pith to reduce the capsaicin content. That way you can get the benefit of the flavour of various peppers while managing the heat.

From the "Wiki page" on Capsacin: 


  Capsaicin is present in large quantities in the placental tissue (which holds the seeds), the internal membranes and, to a lesser extent, the other fleshy parts of the fruits of plants in the genus Capsicum. The seeds themselves do not produce any capsaicin, although the highest concentration of capsaicin can be found in the white pith of the inner wall, where the seeds are attached.[5]