[_wo6a]

They don't impact the ability of the reader to eventually understand the message, but they do decrease the speed at which you can do so if you read any faster than purely phonetically. When your reading begins to involve visual pattern recognition at the word level as well as some degree of pattern recognition of higher-level structures, an error like that is an annoying distraction.

For writing where more time is spent reading it than typing it, it indicates inconsiderateness not to apply the necessary amount of spelling and grammar checking if the writer is unable to produce well-formed writing without specifically attending to its well-formedness. (Although it could also be a status signal to be able to make errors.)

(It's the same reason for code formatting and style rules - they standardize the low level details so readers can focus on the higher-level meaning.)

Any native English speaker who reads much at all (not to mention writes!) and has above a certain level of innate linguistic intelligence will also automatically develop perfect grammar and perfect or near-perfect spelling, so making frequent errors also indicates a certain semi-literacy.