Why have raised beds at all?
The main reason is to allow soil structure to develop naturally without being walked upon or dug unnecessarily (naturally some crops, especially potatoes - unless you use a no-dig method - have to be dug up). I find they make it easier to organise the plot and to manage crop rotation. I am using straw on the paths in-between, so I can walk round the beds whatever the weather. That little bit of height makes access a easier and improves drainage, it also makes it easier to apply crop protection - such as fleece or netting.
I made the beds about 7 years ago using untreated wooden boards from a builders' merchant. They worked well, but by last year were badly rotted. This winter I replaced them with scaffold boards (scaffold companies will supply retired boards - for a charge). These are also untreated - as I don't want preservatives leaching into the soil.
Fixing them together
Last time I hammered in corner posts and nailed the boards to them. The posts were the first thing to rot. This time I have used no posts, but instead joined the boards with metal brackets. I also laid them on damp proof membrane - no idea if that will help slow rotting, I just thought the membrane would reduce the rising damp.