I know I do a lot of spelling mistakes. Over the years I have learned to spell most words correctly and whenever I'm not sure I usually check the correct spelling. So most, if not all of those typos are accidental.
Sure I could (and I do, to some extend) use spell checker, autocomplete functionality or any other tools that could fix or point out those issues. But they aren't without their own problems either. They don't always offer the word I intended.
Occasional typos might make my text harder to read, or appear less professional and high quality. Of course I pay more attention the more important the text is.
Typos with one letter of are usually obvious. It's easy to spot and usually also easy to see what is the correct word. But when the whole word is wrong it might be harder to realize it's actually a mistake. Sure, sometimes it's still easy to spot from the context. Other times a wrong word in wrong place could however change the meaning of the whole text.