Expert flyer is a geek tool.
It's not really about picking a great seat. It's about setting an alert so you can grab the seat if it becomes available. Some pax cancel or change their flight. Some pax upgrade to a higher cabin. Some airlines hold back some seats until closer to departure.
It's about knowing, to a point, how many seats are available in different fare buckets. Using the tools to increase your chances of booking award seats. Increasing your chances of.a free or paid upgrade to business class.
There are free tools, mentioned by PP, to help identify good seats on your flight.