Why > and >> buttons?
Well, it depends on what program you are opening and on how it responds to having a bunch of files dropped on it. You need to experiment and decide for yourself.
First understand that >> sends all the files to the other program as a single shot, in other words a command line which looks like "Nixon Capture NX2 1234.nef, 1235.nef, 1234.nef". Meanwhile > sends one file at a time, so it needs to generate a series of independent command lines like "Nixon Capture NX2 1234.nef" , "Nixon Capture NX2 1235.nef", "Nixon Capture NX2 1234.nef".
With some programs it makes no difference and you can use either button, but in other cases you may want to use >> or > because of how the other program responds and what you want to achieve: