InCD makes your re-writable media (CD-RW, DVD-RW, DVD+RW) behave as a big floppy disk, letting you write files directly to the disc from any Windows application that lets you write files to your harddrive or floppy, including giving you the ability to drag-and-drop files to the disc from Windows Explorer and using Copy/Paste.
InCD doesn't work with recordable write-once media (CD-R, DVD-R, DVD+R), although some competitors software do (Sonic DLA, Roxio DirectCD/Drag-to-disc).
InCD is what is known as "packet writing" software, where Nero Burning ROM is "mastering" software. In general, packet writing software can be incompatible with some other CD/DVD burning software, so if you don't need it - don't install it. OTOH it might work just fine on your system, dpending on what other software you have installed.