Used to compare one thing to another.
More poetic and often used in similies.
듯이 can just be added to any action verb or descriptive verb without needing the modifiers.
When using the noun modifiers they're more commonly used with action verbs. The future tense is only able to be used with action verbs.
When used with nouns it will only use 인 듯이 but it almost never happens, instead use 처럼.