Featherboards are like extra sets of hands in the wood shop. They are generally used to hold smaller work pieces safely in place while feeding the work piece to the blade, typically for applications on router tables and table saws.
This is a miter sled that I built because I was sick and tired of cutting miters that had gaps. As you can see now, no gaps in this new miter. I build frames for posters, so I had to make it pretty large.