A proper disk cache in the operating system will usually obviate the performance motivation for a RAM disk; a disk cache fulfills a similar role (fast access to data that is notionally stored on a disk) without the various penalties (data loss in the event of power loss, static partitioning, etc.). RAM disks are, however, indispensable in situations in which a physical disk is not available.
Another way to use RAM to store files is the temporary filesystem. The difference between temporary filesystem and a RAM disk is that the RAM disk (/dev/ram0 etc.) is fixed-sized and acts like a disk partition, whereas the temporary filesystem (/dev/shm; in Source Mage GNU/Linux also /tmp) grows and shrinks to fit the files put on it.