These days most DJ software and even the higher end mixers or CD players offer built in effects and samplers. These samplers and effector units can sometimes do the trick, but there are also external effects units out there which will take your DJ sets to the next level by giving you advanced effect and sampling capabilities.
The top manufacturers in mid to high end effects and sampler units are Pioneer and Korg. Both offer their own take on effects units and both have their strengths and weaknesses.
If you are an advanced DJ looking to expand your skills and the variety of your DJ sets then be sure to look into an external effect unit for your setup.