You have a definite thing in mind and want to say someone counts it as a different category.
序 · The move
1name the one doing the reckoningwhose view assigns the label?
2把 + the definite objectis this a specific, known thing, not 'a' thing?
3當/看/叫 + 作/做is the verb a reckoning verb, not a verb that physically changes the object?
4name the new category after 作/做does the label go onto the object, and not the object onto the label?
例 · Examples
1爺爺把take hold of (fronts the object)旅行travel (the definite thing held)當作reckons it as, assigns it to退休之後最重要的活動the most important activity (the new label)。
Grandpa treats travel as the most important thing he does after retirement.
The result complement lands the object in a new physical state the action produces (洗好了). 作/做 changes nothing in the object; it relabels it in someone's reckoning. Travel is not altered, only counted as the main activity.
是 states a flat equation the speaker asserts (旅行是活動). 把…當作 puts the equating inside a subject's view and fronts the thing being judged; it is a stance taken, not a fact stated.
indefinite object: 他把一個朋友當作家人 → 他把那個朋友當作家人
label and object swapped: 他把家人當作朋友（reversed sense） → 他把朋友當作家人
physical verb in the slot: 他把朋友幫作家人 → 他把朋友當作家人
English 'regard/treat/take X as Y' puts X right after the verb; learners drop 把 and try 當作旅行…, losing the fronted definite object that 把 requires.