Most of the times we assign the value of an expression to a variable, which uses the variable itself. For example
A = A + 1
increments the variable A. To ease programming ScriptBasic allows the construct
A += expression
instead of
A = A + expression
This is a well known and widely used form by many languages, well readable, though not BASIC like. Likewise programmers can write
A -= expression instead of A = A - expression
A *= expression instead of A = A * expression
A /= expression instead of A = A / expression
A \= expression instead of A = A \ expression
A &= expression instead of A = A & expression
This is more readable for most of the programmers and results slightly faster execution for addition, subtraction, multiplication, division, integer division and string concatenation respectively.