Evaluate[expr] causes expr to be evaluated even if it appears as the argument of a function whose attributes specify that it should be held unevaluated.
Example: Hold[Evaluate[1 + 1]] .
You can use Evaluate to override HoldFirst, etc. attributes of built-in functions.
Evaluate only overrides HoldFirst, etc. attributes when it appears directly as the head of the function argument that would otherwise be held.
See The Mathematica Book: Section 1.9.1 and Section 2.5.5.
See also: ReleaseHold.