I agree. Written language is generally more formal than spoken language. Think about when each of them is used.
Written language is used when you want to make sure your thoughts are organized and you're saying exactly what you want to say in the way you want to say it. Politicians use written scripts when they care about getting the wording of their positions exactly right or when a speech is very important. It's also used when you want to make sure there is no misunderstanding, which is why contracts and other legal documents and laws must be put in writing. The language is more formal (no contractions or slang, usually) and the ideas are more structured (organized).
Spoken language is much less formal and generally not particularly organized because it happens in informal settings--and there is usually an audience to hear it. We have conversations with friends, teachers speak informally to classes, successful sales presentations are given without scripts, and any business transactions (banks, stores, libraries) are conducted by speaking, rather than writing. That means sometimes we have to say "that's not what I meant" or "what I meant to say was...." We don't always get it right the first time when we speak, but we do usually get to keep talking until we fix it since the person is right there. Spoken language also has the advantage (or disadvantage) of being accompanied by body language to help the listener(s) interpret what is being said.
Both types of language are important in day-to-day life. Imagine all communication taking place only in writing or only by speaking. I know it was that way once, but it sure would not be practical in today's world.