It lowers the barrier to entry for FPGA development significantly. The Papilio project, for example, tries to build an "Arduino for FPGA" and runs into the problem that a full ISE or Vivado installation and corresponding license is needed. This aggravates the primary problem that the Arduino has solved for microcontrollers, easy entry.

Also: Incremental builds, partial reconfiguration, porting to macOS. I'm not sure which of these Vivado has solved. With an open-source toolchain, those interested in these features at least have a chance of building them themselves.