Most shift registers (like the 74xx595) use TTL (or similar) outputs, which output a positive voltage when turned on. This makes connecting transistors and similar much similar. However, they generally can't provide much current. The shift register you have effectively has the transistor you have wired up inside the chip, so it can "sink" (which is what your transistor is doing) more current - something like 150mA per output (check the data sheet for exact figures). So, if you're not wanting to connect anything that uses more than 150mA then you don't need any of your external transistor, but can just use the shift register directly. The output of the shift register would be the collector of the transistor. So, to use it like a 74xx595 is slightly more complex, but to use it "properly" can be simpler, depending on your application.