There are two common reasons in my experience. The first is that it can give an indication of how extensive the revisions requested are. For example if the editor is only asking for typo corrections and a clearer figure, she might ask for a revision in two weeks. But if she wants more data that's difficult to acquire, she might give two months or longer.
The other reason is purely pragmatic. By setting a deadline, two important things happen in the editorial management system (EMS):
- The automated reminder system is triggered. You can expect to receive automated reminders that your revision is due on ____ a week before the nominal deadline, something that would not happen without a deadline.
- Some authors simply abandon a submission. I don't think they abandon the paper entirely, but they stop caring about the submission to this journal. In that case the paper goes dormant, which clutters the EMS. If a long time has passed since the deadline, then the desk editor can reasonably conclude that the authors are not planning to revise the paper. They can then confirm with the authors and / or just remove the submission from the EMS.
Can you submit a revision after the deadline? Yes - that's why the email says "We assure you that a revision will be reviewed normally even if it is submitted after the deadline." However, if you're going to take a lot longer than the deadline to submit a revision, you should warn the editorial office that you are planning to revise the article, or it might be removed (per #2 above).