What is the purpose of 并 in 他虽然并不聪明， 可是很努力?
並/并 http://www.cantonese.sheik.co.uk/dictionary/characters/844/
[3] [adv] (used before a negative for emphasis) actually; definitely; really

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他虽然不(not)聪明， 可是很努力。
他虽然并不(actually not)聪明， 可是很努力。

Other example of 并 as an adverb before a 'negative':
無(doesn't have) --&gt; 并無 (actually doesn't have)
未(have not/ not yet)--&gt; 并未 (actually have not/ actually not yet)
沒 (doesn't have/ did not)--&gt; 并沒 (actually doesn't have/ actually did not)
並 in '並有'(also have) or '並已'(had also) means 'also' since  有 and 已 are not negative

'並' 不一定 等於 '並且' ('並' does not always equal 'and also')
'並' 并不一定 等於 '並且' ('並' actually does not always equal 'and also')