"Traveling is important."
여행 is a noun, by itself, but you need 하는거 for words that are inherently verbs, to make it into a noun.
Example: 행복 = Happiness 행복해 = to be happy 행복하는거 = (the action of) being happy (followed by a description)
걷다 = to walk 걷는거 = (the action of) walking (noun of verb)
저는 행복해 I am happy
행복하는것은 좋아요 Being happy is good
저는 집에 걸어요 I walk home
집에 걷는것을 힘들어요 Walking home is tiring
Please refer to this lesson by TTMIK. https://talktomeinkorean.com/curriculum/level-3-korean-grammar/lessons/level-3-lesson-10/