They have little pores where vapor bubbles can sit in and get trapped. So instead of the flask bubbling and having its contents spill out violently, the chips will trap the bubbles. It effectively allows you to work at higher temperatures without the solution getting all over the place.
I know this is an old comment but it also breaks the surface tension and avoids a superheated solution. For example, heating water in a microwave is dangerous because nothing is in it to break the surface tension letting the water go past its boiling point without actually boiling. Once you touch it, the water gets disturbed breaking the surface tension and letting the water rapidly boil and causing boiling water to spill all over you.
