It's easiest to see how historical and cultural context affects texts by using a specific example. Let's say you want to write an editorial on censorship in your local school district, but you live in a very small rural town that has a shrinking farm population. Most of the subscribers to the local paper and school board members are in their 50's and 60's because younger people have left the town for greater economic opportunity. What do you know about your readers? Unless they are extraordinary for other reasons, most of them finished high school 35 or more years ago when the curriculum was very different in its emphasis. Many of them may believe that older, traditional approaches to education were more effective than current approaches. (After all, they turned out just fine.)
Your readers may well believe that what was good enough for them in school--including a very limited number of carefully selected books to read--is good enough for teenagers now. If you were to argue that "classics" don't help teenagers deal with the problems they face today and that schools need to stop censoring books that tackle difficult problems, you'd need to be careful about the kinds of evidence you used to back up your position. If you include references to popular song lyrics or certain television programs in your editorial, these readers might not recognize the references or might even be offended by them. So the context of daily life with its moral values, political and economic judgments and geographic/historical/generational issues also plays an important role in shaping an effective message for specific readers.
Similarly, certain companies have a corporate culture that shapes the writing within that context. Reports have to be formatted in certain ways, and certain expressions are well-known and accepted within this small community. Once you become immersed in such a corporate culture, it's easy to write within these constraints. But learning your way around a new culture as a writer can be painful at first because the cultural conventions can make you feel like a "newbie" or an outsider, and in some corporate cultures public ridicule goes along with violating the conventions.