Mistakes on Purpose
When the teacher does all the talking and the students are just supposed to listen, they're liable to lose focus. It's important to keep the learners engaged throughout the lesson. There are plenty of ways to do this, of course, but one way is for you - the teacher - to make mistakes.
Students (depending on their culture) love to correct teachers. Give them opportunities to correct you. The mistake could either be on the topic you're currently covering, or it could involve something the students should already be very familiar with. The first few times you do this, you might want to make it pretty obvious. If your students are shy or are too respectful to correct you, you might want to let them know that there's a mistake in what you've just said or written, and their task is to find the mistake. As the students get the hang of correcting you, make the mistakes less obvious. But don't move past them too quickly. If no one corrects you right away, you may have to pause, forcing them to take a closer look at what you said or wrote. Or just ask them if they agree.
Anyway, here are some reasons that teacher mistakes can be beneficial in the classroom:
Once students come to expect that you might make a mistake at anytime, they'll pay more attention. Lots of students want to be the first one to correct you.
Students who identify the mistake receive a confidence boost.
Correcting you demonstrates a student's understanding. If students can't correct you, you'll need to review that language point again.
If students are on the look-out for your mistakes, they'll be more likely to identify their own mistakes. For the students that don't jump at the chance to correct you, at least they'll get exposed to mistakes that are common for that grammar point; so if they're likely to make that same mistake themselves, they might acknowledge that something about it isn't quite right the next time they do so.
Of course, once the mistake is addressed, you'll need to clearly fix the mistake. In fact, a student shouldn't simply point out that something is wrong, but they should be able to explain to the class why it's wrong (or conversely, why their correction is right). But with smaller, simpler mistakes, you can make the quick adjustment after half the class shouts out their response, then just move on.
Also, keep an eye out for students who didn't respond to the mistake; if they seem lost, you'll need to address their confusion before moving on. After all, mistakes-and-corrections aren't a gimmick; they're a teaching opportunity.
Making mistakes on purpose can retain student attention and can help their own language development and problem-solving skills.