no you don't NEED it but the only thing you may wanna get is the bracket if you'd like. but me I just used 3M double sided tape and it's been holding up for 9months now with no problems

 

the HID harness is for hooking it up to like atv an stuff with smaller gauge wire than trucks. the adaptor is for the 9006 bulbs, the error eliminator is for if there flickering I believe (not 100% sure) and the brackets are up to you if you want them, there $4.