Incorporation of a company under a particular name does not prevent others from selling goods or providing services under the same (or a similar) name.
A company name serves only to identify a legal entity. It does not confer a proprietary right in that name. A trade mark registration creates a legal right which may be relied on to prevent another party using a name the same as or similar to that registered as a trade mark.
A registered trade mark is something separate from a company name. If a company is incorporated under a name which includes a registered trade mark and that company provides similar goods or services, the owner of the registered trade mark may compel that company to change its name.
Moreover, if a company uses any part of its name as a trade mark on or in relation to certain products or services, and another party has registered the same mark or a similar mark in relation to those products or services, then such use by the first company of its name may constitute trade mark infringement.