I think that it is used for the same reason that all the other Arabic numerals (1,2,3,4,5,6,7,8,9) are used in the Chinese language--since they are used internationally (and in trade). 0 was modified to 〇 for æsthetic purposes/in order to take up the width of a character, I believe.
It is also a visual symbol/ideogram. There is nothing inside the circle, therefore it represents nothing/zero.
PS. Oh, I think you mean why it is sometimes used in conjunction with Chinese numerals (二〇〇二年, etc). I imagine it is for legibility since 〇 can be read easier from afar. I'll let Chinese speakers confirm for this one !
1, both 〇 and 零 are Chinese and both are read as líng. Just as you said, it´s some special and seems non-Chinese;
2, 〇 only appears in numbers, expecially for years and pages, which indicates a blank, ex: 二〇一二年. You can simply understand it as a Chinese form of Arabic number 0;
3, In other situations, you should use 零. Also, 零 has many other meanings 〇 doesn´t have.