This gist contains examples of why thunking can be useful

excel-lambda-thunk-illustrations.txt

// This takes several seconds to evaluate
huge_array = MAKEARRAY(10000,5000,PRODUCT);

// By putting the array in a thunk, we can choose not to evaluate it
thunked_huge_array = LET(my_thunk, LAMBDA(MAKEARRAY(10000,5000,PRODUCT)),my_thunk);

// We can evaluate it by putting () at the end of the LET function's return value
evaluated_on_LET_return = LET(my_thunk, LAMBDA(MAKEARRAY(10000,5000,PRODUCT)),my_thunk());

// Or just putting () outside the LET function
evaluated_on_LET_result = LET(my_thunk, LAMBDA(MAKEARRAY(10000,5000,PRODUCT)),my_thunk)();

// This will evaluate the array even though it isn't used
LET_is_not_lazily_evaluated = LET(
    huge_array, MAKEARRAY(10000,5000,PRODUCT),
    "I don't want that array"
);

// IF only evaluates either 'then' OR 'else', not both
deciding_with_IF = LET(
    my_thunk, LAMBDA(MAKEARRAY(10000,5000,PRODUCT)),
    IF(Sheet1!A1="Do it!", my_thunk(), "Didn't do it")
);