Privilege escalation – If there is a security vulnerability that’s exploited (in say, your web browser), by not running your programs as root will limit damage. If your web browser is running as root (because you logged in as root), then any security failures will have access to your entire system.

Why is it that it is not advisable to use root as the default login?

The default root login opens up every possiblity to execute harmful steps like deleting important files, copying, hacking or system crash itself. … Thats why it is not advisable to use root as the default login.

Why should you never run as root all the time when on the system?

If you run a program as root and a security flaw is exploited, the attacker has access to all data and can directly control the hardware. For example, it might install a trojan or key-logger into your kernel. In practice, though, an attack can do a large amount of damage even without superuser privileges.