I find it kind of weird since a sophisticated drawing style suddenly becomes very cartoonish/simplistic. It's like a break. And the features of the face change. I noticed this in a couple of series now
If you have access to Scott McCloud's Making Comics manga section you can read a long essay on this.
To sum it up, Japanese comic's drawing is much more subjective than their traditional European and American counterparts. Where those put emphasis on character poses and their location on the scene, manga highlight the feelings told on facial and body expressions.
Japanese comics are usually purposely unrealistic in this style, even when the manga is realistic, serious and everyday themed. When a character is having intense feelings, his eyes sparkles, when he is shaken by bad news a thunder appears behind him and his eyes and pupils vary in size depending on the emotion.
Continuing with this tradition, the super deformed (SD) style was introduced in mangas and quickly adopted. Characters' expressions are exaggerated, the body shrinks and the head is enormous, normally expressing some intense emotion (probably anger, but many others). This is used even in serious comics, usually as a comic relief.
Why? It must be understood in the previously mentioned subjectivity of manga and anime. It has no story meaning (even if the character picks a big hammer), the change of body proportions don't happen in reality, it only counts as the expressed feeling.
Note that even when the manga or anime don't use SD style, the style, the body proportions and the face are usually somewhat flexible and subject of emotions (instead of super-deformed they are just deformed). Compare to many other forms of subjectivity, like the one dealing with time or distance.