Both of these are fine:
1. I often watch movies. <pre-modifies>
2. I watch movies often. <post-modifies>
As you know, the rule is to place only before the verb, as in 1., which isn't to say that placing it at the end of the sentence, as in 2., is wrong. It's not wrong. Speakers do it for several reasons:
3. I watch movies (very) often.
- if it stands for a larger phrase, say elliptical (very) often,
- for emphasis or
- as an afterthought:
4. I very often watch movies.
5. I watch movies often.
6. I watch movies--often.
Student or Learner