Basically, if you want to store information, you need something like code on encryption. RNA stores genetic information quite similarly to DNA - encoding it in a nucleotide sequence. Probably, the fact of the matter is that ancient RNA was successfully serving as a genetic information storage during the processes connected with the origin of life in the stage of RNA world. Some organisms now even have a RNA genome, for example, viruses. But the main mechanism remains the same - genetic information is encoded by a nucleotide sequence. However, RNA also have many other important functions, which are somehow connected with genetic information storing, including its role in translation, processing etc.