再一次地, why is 地 used?
In Chinese「地」is the adverb marker. 
It's used bewtween an adverb and the verb it describes.

The phrase「再一次」 acts like a adverb whcih means "do something once again".
So use 「地」after 「再一次」 and the verb behind it。

Example：
「再一次地感谢大家」 --> Thanks everyone once again.

And 「地」can be omitted in this case, so
「再一次地感谢大家」 --> 「再一次感谢大家」
This kind of usage is usually more informal.

The words 「的/地/得」 is quite confusing in Chinese language.
Even Chinese native speakers do not care about the difference between them.

More information please see:
http://eastasiastudent.net/china/mandarin/%E7%9A%84-%E5%9C%B0-%E5%BE%97-summary