Golden Temple as it is known popularly is important to Sikhs as it is the first Gurudwara(Sikh place of worship) to be built, also it was in Golden Temple that for the first time Guru Arjan Dev Ji installed Guru Granth Sahib Ji in the year 1604 A.D.Further more, at Golden Temple there is continuous kirtan (Singing of Hymns) everyday.Also, from here the Sikh's cultural tradition also moved on. Therefore, Sikhs visit Golden Temple. 
Please note there is no religious importance to it as anyplace in the world where Guru Granth Sahib Ji is that is equally important as well, and as a religion Sikhism don't have any Holidays or Holy place but as mentioned above since Golden Temple happens to hold historic and cultural importance to itself, Sikhs visit it. 
Also, Sikhs in themselves refer Golden Temple as Harmandir Sahib that's what the actual name is.