-인 것처럼 is a more detailed expression because it has 이다 in it.
처럼(like) is used for simple phrases (like NOUN), whereas -인 것처럼 constructs a full "as if / like" clause.
- 쟤는 유명인처럼 행세하며 다닌다 = He goes around acting like a celebrity.
- 쟤는 자기가 유명인인 것처럼 행동한다 = He acts like/as if he is some celebrity.
For example, this sentence is difficult to express without -인 것처럼.
- 그것이 나를 보는 마지막 순간인 것처럼 슬퍼했다 = (He) was saddened as if it was the last moment to see me.
- 그것이 나를 보는 마지막 순간처럼 슬퍼했다 = BAD (When you have a subject and verb in the clause, you need -인 것처럼 to complete the clause)