"He changes girlfriends every year."
都 is used when it comes to quantity or doing something every time. Of course, in English it is unnatural to read the whole sentence with 都, using "all". However, this is the common practice in Chinese language.
Without including 都, the sentence becomes more time-generic. Then, the meaning of the sentence can possible indicate that he doesn't change girlfriends in one of his previous years. For instance, from 2000 to 2010, it is possible he didn't do that in, say, 2002. Thus, sentences without 都 is less specific than those with 都.
Otherwise, if 都 is included, then the sentence indicates that he does change the girlfriend each single year.
Note: 都 does not necessarily mean one has to perform the certain action since their birth.