DVD-RAM discs are used primarily for critical data as they are rather expensive. They can be made to function as a sort of hard drive but as HD space is so cheap and much faster, most people don't use them for that. They last much longer than normal discs and they verify data as it is written.
Also, some Panasonic stand alone recorders only use RAM for re-writable and then they copy to a PC for editing and final burning to a regular disc.
Here is a good source. BTW, if a disc is 3X it will only do 3X even though the drive is 5X. Get the 5X if you want thet speed. These are very slow to write as they verify all data so 5X is worth the difference.