It is proper Netiquette to ask before sending attachments. Don't send attachments without permission.
Not all email addresses are created equal. Many different companies and people take additional security measures to ensure email messages do not include viruses. Email servers may block attachments, large files, and various other things.
Since there are so many different things that companies can do to block viruses it is good netiquette to ask before you send emails with attachments because blocking attachments is common.
Ask before sending attachments because many servers block attachments and large files.