In iOS 10, I used to tell my clients that if you have a paid app, it makes sense to have a video because people would want to see the app before there is a barrier to entry — they have to make a payment and its hard getting a refund. But for free apps, if you have the money then go ahead and spend it for video but if you don’t, you don’t need to push it.
In iOS 11 though, the way that Apple has incorporated video, I believe it’s worth investing a few dollars in your app video. If you will notice, when you search for app, you will now see certain things moving because when an app has video, Apple is auto playing it. This catches a user’s attention versus something which is just static. As humans, we are more attracted to things which moves so a user can already see bits and pieces even if he is just scrolling through the preview. So whether you have a free or paid app, it’s important to incorporate video as it can lead a potential use to click ‘more’ and check you app out.
If you are on a budget, there is an app called ScreenFlow where you can create multiple videos and it works well with Mac. You can play around and create your app preview video by yourself using this but if you want to hire a professional, you can check out the guys at Apptamin.