DJ software has more functionality, and let you do new things that were just not possible on vinyl or CD. It is used by DJs to replace their turntables and CD players, and use digital music instead of vinyl and CDs.
They need special controls like crossfaders, loops, effects, etc, in order to “mix” the songs. Because being a DJ is more than just pressing play, DJs cannot just use iTunes or Media Player to perform a mix. VirtualDJ is the application that DJs use when they want to use a computer to play their music.