The Complex element is a group of elements that you can save and use it later anytime. For example if you use often image and text next to each other, you may save that structure as a Complex element, then next time you will not have to build it up again from scratch using Column, Image and text elements.
Another good example would be round-cornered columns. These are not easily constructed – you have to put together different background images for corners in Columns to have it responsive – thus, if you save it once, you will be much happier to be able to just reuse it anytime as a whole.
The Complex element can save you a lot of time spent with repetitive work, and on top of that, it works between templates. So if you want to copy some parts of a template to another, this element is the tool for that.