Do the people write different than they speak? A lot of things that we use in our spoken language we do not use in our writing and vice versa.
There are a lot of differences between our spoken and written language. One difference is about timing. Written language can live through time as long as the language is still understood. For example historical documents like old letters that are found and are still understood . Our speech is usually used to communicate at that time. Another difference is in making the information clear for people to understand. While speaking usually you are receiving feedback. It helps you to know what things were unclear and maybe what things you need to paraphrase for it to be clear.
You can do it and change some ideas while you are talking. As to writing it is totally different. You need to make sure everything you write is clear and people can understand the message. Written language is much more complex that spoken language.
When you write you need to make sure the punctuation is right, the sentence has correct structure. The sentence should not be too long. Also the paragraphs should be clear and flow one to another. As to spoken language we use tone, volume, pauses and timbre. Another difference is that we do not speak as formally as we write, except for messaging friends or family. When we right we do not use slang, we write properly with good and complex vocabulary. When we talk we use more informal language.
Both written and spoken languages are very important, also they are very different. We need to know the difference between what we can write or say and what we can not, so that it will be properly .