You may not actually need root permissions to install a given software package, or you may not have access to root privileges via
su or other methods.
One of the
./configure script's common options is
--prefix, which changes the base installation directory from whatever the default is. So on a system where I have no root privs at all, I can still install this software to my own home directory:
./configure script has lots of other options, some common to all, some dependent on the particular package you're compiling. Use
./configure --help to see them all.