RO vs RODI water

DI removes any remaining TDS (total dissolved solids) that the RO membrane leaves behind. The TDS of the incoming water may or may not necessitate the need for a DI filter depending on the type of aquarium you are setting up. A high rejection (98%) membrane can do a pretty good job of filtering the water on its own. My incoming TDS isn't terribly high and the RO membrane alone gets the TDS down to 1 or 2. I still run it through a DI cartridge that takes it down to 0.

A full blown reef tanks with lots of SPS, LPS, and other sensitive corals will probably benefit the most from pristine 0 TDS RO/DI water. A softy tank will probably run just fine with low levels of TDS. I don't think a fish only tank would require anything beyond RO water.