In occult thinking the circle represents a boundary or an attempt to control the power of the symbol. Gay Symbol - a pink triangle in a circle is used by gays to let other gays know their homosexuality, but it is also used by some Satan worshippers to conjure demons directly from hell. The circle represents a boundary of power so they don't get hurt by the demon. Some use the triangle without the circle because they want to be possessed. Those are Max's comments, but please understand that none of this is absolute.
Symbols change over time and have different meaning from one group to another.
This is only a generalization. Kabbalists popularized the use of the symbol as a protection against evil spirits. The Jewish community of Prague was the first to use the Star of David as its official symbol, and from the 17th century on the six-pointed star became the official seal of many Jewish communities and a general sign of Judaism, though it has no biblical or Talmudic authority. The Magen David gained popularity as a symbol of Judaism when it was adopted as the emblem of the Zionist movement in 1897, but the symbol continued to be controversial for many years afterward. When the modern state of Israel was founded, there was much debate over whether this symbol should be used on the flag.