Reed-Solomon and similar codes correct for bit errors in a particular block by transmitting extra bits in each block. Fountain codes correct for missing blocks by transmitting extra blocks.

If you just used RS, the only way to overcome really long bursts of errors (thousands or millions of bits long) would be to either to request acknowledgements from the receivers and retransmit unacknowledged blocks. If acknowledgements weren't possible, you would need to transmit everything multiple times. And, if you had enough receivers, all missing random packets, you would end up retransmitting everything even if you had acknowledgements. By using fountain codes, you can transmit an extra few percent of blocks and still succeed in the face of long dropouts.