The triangle is actually what contains a being and is the protection from the magician from the ebing, the divine names enclosing it are the protection.
The circle is to contain the energy raised in the rite and to prevent outside energies from leeching off the operation. If one reads the more modern renditions of manuscripts of evocation which are better researched than the usual Mather's/Crowley goetia which everyone seems to follow, then it will be found that the triangle of art rests inside the circle.
The circle is necessary if you are using that system, if not, then it is debatable and up for experimentation. If you don't feel confident in your ability to handle energies summoned and banish accordingly, then you aren't by any means ready to summon/perform evocation.