The 5400 RPM hard drive that comes with the PS4 and PS4 Pro is bone-chillingly slow at this point, so an SSD will be a nice upgrade. It won’t make loading times instantaneous, but it can decrease them ...
Straight off the bat the biggest improvement that you’ll get by stuffing an SSD into your PS4 is improved loading times. Simply put, just about every game you own will load much faster from an SSD.