JEE Paper 2 is for admission to B.Arch and B.Planning courses. For admission to these courses in IIT or NIT, if you qualify in JEE Mains, you have to write JEE Advanced. So if you dont want to take admission in IIT or NIT or GFTI, you need not write JEE Advanced. But if you want admission in B.Arch, you have to write JEE Mains Paper 2. NTA will conduct JEE Mains twice every year once in January and once in April. So if you missed it in January session, you can apply for April session.