Why sexist language matters?
The question is not well phrased, as it lacks context and specificity. Having said that, I will attempt to answer it.
Sexist language matters in the writing of history for largely the same reason ethnicity or nationality matters: the distinctions between the genders can be reflected in the emphasis placed on different eras or subjects within an era. A logical example would be the history of the Suffragette Movement, the struggle by women for the right to vote in elections. A history of that movement written by a male might differ substantially from one written by a woman, in that the latter would almost certainly feel emotionally closer to the issue by virtue of her gender, and her version of the history would reflect that perspective. A male, not having been subject to discrimination on the basis of gender, might produce a good draft in terms of the basic facts, but might lack a deeper relationship to the subject that would result in a different presentation.
The question of whether and how the genders differ in how they perceive various subjects has been examined through surveys. In many issues, for example, the nuclear arms competition between the United States and the Soviet Union, these surveys, along with voting patterns, have repeatedly demonstrated that such differences of perception exist. Those differences are reflected in the writing of history books or articles. Male scholars or writers have often been criticized for perceptions of gender-biased analysis. Many social issues divide strongly along gender lines, and sexist language is often found in treatises on topics like reproductive rights and children's issues.