Writing sales or advertising content is the standard practice of copywriting. You may be producing email text that promotes your website or generating sales pages for website owners. 

Writing marketing materials, brochures, or other texts geared toward converting readers into customers or leading them to take action (like joining an email list)

Working as a copywriter requires more specialized skills than being a freelance writer, but you can also earn more money. As a freelancer, you can offer copywriting skills you have gained through your job.