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, āto work outā means to exercise (typically with the goal to be fit or develop certain skills or physique; more intentional/goal-directed than the meaning of āexerciseā). This is completely different from āto work.ā
Other times, adding āoutā does not change the meaning as drastically; for example āhelpā and āhelp outā mean close to the same thing, although the usage is somewhat different. āHelpā can be modified by a following verb, as in āhelp me make dinner,ā whereas āhelp outā is more general and stands alone, as in āI came to help you outā (you cannot say, for example, āI came to help you out make dinner.ā)
As Clara (Blue) said, the phrasal verbs simply must be learned because they are essentially a different word.
Hope this helps! :)