As we know that root is superuser in linux, having 
unlimited power to manipulate the system: when logged as 
root, you can read,modify or delete at any file,change into 
any directory,modify configration of the system & perform 
almostany task on system.
AS root user you can do virtually unlimited damage to the 
system, corrupting oor deleting all files. So,it is 
essential that you not log in as root user unless it is 
absolutely necessary.
Other reason for it for secuirty purpose,if you work on a 
linux server & if it is hacked by a hacker in this case if 
you log as root,hacker can acquire the power of root user & 
may try to damage your system. So during first login of the 
day try to login as normal user rather then root user, 
after that you may login as root user.