Reason to add a name of the theme like (‘menu-1’ => __( ‘Primary’, ‘twentynineteen’ ),) in PHP?

The name you see is the text domain of the theme. __('some text', 'text-domain') is a translation function, which makes it possible to translate the some text into different languages. There are also other translation functions that can be used in different situations. You can learn more about translations on the codex, I18n

P.s. if the text domain is left out, the function uses default WP text domain and tries to find translations from that. For example simple strings, like yes and no, usually gets translated from the default translations.

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