Some databases will give you separate search boxes. For instance, databases hosted on the EBSCOhost platform.
This allows you to search without brackets. All words linked with OR for one aspect of a search go in the same box. Here's an example:

In a database where you are offered only one search box, if you are using both AND and OR in the same search, you will need to use brackets.
If you were to express a search in the same way as below, your results would include records that mention film and horror, and then records that mention thriller not necessarily in combination with film:
However, using brackets, as shown below, ensures that your results are comprised of records that mention film with either horror or thriller: