The short answer
Yahoo! JAPAN is a search-and-portal platform in Japan you cannot ignore alongside Google. Its search ads resemble Google's, but its distinct display inventory and audience warrant separate operation.
Beyond search, Yahoo! JAPAN carries large portal traffic across news, shopping, and mail, giving its display network (YDA) wide reach. Rather than cloning Google campaigns, adjust bids and creative to Yahoo's inventory.
Its audience skews somewhat older than Google's, so for some categories Yahoo can convert more efficiently. Test both with the same conversion definition and allocate budget by evidence.
Keywords and content require localization, not translation. Keyword design that reflects Japanese search intent and script variants (katakana, kanji) drives results.