I am as simple as a circle, worthless as a leader but when I follow a group, their strength increases tenfold. By myself I'm practically nothing. What am I?

Why this works

This riddle plays with the concept of the number zero and its unique properties in mathematics. The phrase "as simple as a circle" refers to the shape of the numeral zero, which is indeed circular. The line "worthless as a leader" suggests that alone, zero holds no value; it represents nothing. However, when you place it after a number (like turning 1 into 10), it significantly increases the value tenfold, showing how it enhances the strength of a group of numbers when it follows them. Thus, zero embodies both the idea of nothingness and the power of multiplication when in the right position!