I can follow the assumption that the brain has evolved in order to help survive. I have a hard time wrapping my mind around how it has anything to do with reproduction.
The key for me is in the word "evolutionary". Clearly the focus on this branch of psychology is how we as humans have developed in our thinking from our ancestors. In particular, special emphasis is placed on how our psychological characteristics today evolved in response to our environment, and especially how we have developed behaviours and characteristics that relate specifically to our survival as a human species.
Evoluntionary psychology is the study of man's present day psychologyical traits in relation to those from our distant past. Evolutionary psychologists attempt to show that our psychological traits evolved as adaptations to our environment, most notably, the psychological adaptations related to behaviours that promote survival.
The biggest example today is that of racism. In our distant pastm, fear of the unknown helped us to survive by our reticence to take unnecessary risks. That animal you've never seen before, that tribe that looks different than you, that berry you never new existed - all could be friend, but then, all could be foe. The safer assumption for survival is to assume the unknown is foe. For that reason, today evoluntary psychologists would explain racism as that evolutionary fear of the unknown, which activates in us a sense of fear. Fear can lead to hatred, and biogtry, and ignorance, instead of understanding. Today many are able to overcome that unconscious drive to fear the unknown when it relates to such superficial characteristics of people, but not all have.
The field is basically putting into context things that you are doing in relation to how you did them before or how people did them before. It helps to explain the need to do things or the desire to do them and in what capacity would you do them and why.