Common ground for different voltages
it's bugging me because I'm essentially connecting Vin- and Vout- on the converter and if that was okay then why would they even have put Vout- on there?


You probably have an isolated converter.

If you need to maintain galvanic isolation between the input and output (for example, for user safety), then you would not connect VIN- and VOUT-. 

If you don't require isolation, then it's perfectly okay to connect these together.

By providing separate terminals, they give you the choice whether to use the converter in an isolating configuration. If they only provided one COMMON terminal, you'd have no choice and wouldn't be able to isolate input from output if you wanted to.

If your converter is not an isolated design, then they might just provide separate terminals for convenience in wiring.

Edit The photo you shared is almost certainly not an isolated design. They are probably providing separate input and output terminals for your convenience only.