Owning your domain name, only means you can build a website for your business, but as far as trademark is concerned, the name is still available for anyone to go and file it as that filing is done before the Trademark Office of interest and grants exclusive rights for 10 years (this is in most countries. However, there are countries that provide less or more years).
By filing your trademark, you have rights to protect your business as a whole, that means nobody else can use that name to confuse your clients, and if they do, you have the legal tools to pursue them and win, something that just owing a domain name will not give you.
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