Renko bars don't paint a color unless the market moves a certain amount based on its settings. When the market moves up it turns green, if it moves down it turns red, simple color changes alone can say a lot. They're a good way to try to find trends somewhat objectively and seem to be a good way to eliminate time and can replace other time-based indicators that can whipsaw or lag. The bars have a tendency to repeat themselves so it's a good way to find trends. There aren't too many settings for the box size, most people either just choose 5, 10, 14, etc where as other indicators have many options that differ on different markets. The numbers can be chosen easily enough to pick a sweet spot with just a single input where other indicators such as have multiple inputs to pick the right number that can make it difficult to choose from(although it won't be as precise as a would sometimes but can be worth the objectiveness and consistency and same setting repeatability in different markets in my opinion). Some example strategies could be to use them as an alternative trailing stop, finding trends, a simple color change for entry and exit on top of other strategies, etc. It can do the job of many in an all in one price action type indicator(although not better all the time, it can come close enough). Despite all this, it does seem to depend on which time-frame it's being looked at, how TV does the calculation for it, and how one can use this with the lack of practical information on it out there.
In true TradingView spirit, the author of this script has published it open-source, so traders can understand and verify it. Cheers to the author! You may use it for free, but reuse of this code in a publication is governed by House Rules. You can favorite it to use it on a chart.