do you need a hardware mixer if you have a DJ Controller?

Depends on your controller and where you play at…
If you have a weak controller and a club setting, you may need a hardware/club/professional mixer,
Why ?
1. So it wont put too much pressure on your controller, ya know the peak volume UV meter in your controller (hence if u had on it)
2. Put the pressure on the hardware/club/professional mixer, ya know turn up their master volume, turn up their trims
3. Some controller / software doesn’t possessed FX, most of the time the hardware/club/professional mixer do
4. Some controller may not have those extra plug-in for other source, ya know in case your laptop screw-up or other technical issues, you left out with no backups, leaving the dance floor with this silent awkward sensation

In other setting (small event and such), you may not need those hardware/club mixer,plug those sucker straight to the amp,
although some band mixer be good also (ya know them BIG bold huge mixer with lots of buttons and faders)
Just make sure your controller comes with nice kick ass sound card (be it internal or external sound card)

Yep, I tried to answer your question in respect to a simple 1 DJ, 1 Controller/Laptop, 1 PA environment. Rizki’s point are valid, but were a bit out in left field as far as your question was concerned in my humble opinion.

I have, at times, hooked up my controller to one of my live sound mixers. Usually it would be at events where I would have to do some microphones and/or small acts and do DJ stuff in the breaks or after the performances. Obviously it works, but it is by no means necessary. When I go out on any of my mobile DJ gigs, I never bring an extra mixer. Just the controller is sufficient.

Greetinx,
C.