Hey,

JEE main and JEE advance require  to get admission in IITs or NITs. So if you want to get admission in IITs or NITs you have to score within cut off to get IITs or NITs. AAT stands for Architecture Aptitude Test  . It is taken to get admission in college like IIT kharagpur , IIT Roorkee, IIT varanasi (BHU) only. So if you want to take admission in these three colleges for b. Tech architecture then it is mandatory  to score good in AAT. For more information regarding AAT registration, syllabus and other things go through this link:-

https://engineering.careers360.com/articles/jee-advanced-aat .