It does. It matters a lot.
Some people think worrying about spelling and grammar is anal and backward. They are wrong.
There are two reasons why spelling and grammar are important and will remain important for as long as people still read printed words:
First: Well-written, properly-spelt, grammatically-correct English is unambiguous.
Poorly written English is open to misinterpretation.
If being understood is important, then worry about spelling and grammar.
Second: Well-written text flows, it’s a pleasure to read. It sends readers a message about your professionalism and wisdom. It is credible.
Poorly written English jerks around, it causes readers to stumble. They may not realise why this sets off alarm bells in their heads, but it does.
Too much poor English and they’ll question the message. This may not happen on a conscious level. It may not happen with all readers. It will happen enough for it to matter.