Some of them create a new meaning (becomes a phrasal verb) - in other words, when put together the two or more words act as a completely different verb. For example, âto work outâ means to exercise (typically with the goal to be fit or develop certain skills or physique; more intentional/goal-directed than the meaning of âexerciseâ). This is completely different from âto work.â
Other times, adding âoutâ does not change the meaning as drastically; for example âhelpâ and âhelp outâ mean close to the same thing, although the usage is somewhat different. âHelpâ can be modified by a following verb, as in âhelp me make dinner,â whereas âhelp outâ is more general and stands alone, as in âI came to help you outâ (you cannot say, for example, âI came to help you out make dinner.â)
As Clara (Blue) said, the phrasal verbs simply must be learned because they are essentially a different word.
Hope this helps! :)