If one keeps the bike speed the same and changes to a lower gear, the gear ratio has changed. One needs to pedal faster as each revolution of the pedals does not send the bike as far. Each revolution does not do as much work. This reduces the force needed to rotate the pedals.
This can be great for your knees and for more power. Or for long rides and not wearing out your legs. But you can take it too far also and spin too much. If you can't spin comfortably in a certain situation a higher gear may be better for that situation for you. There are all sorts of reasons to spin, or not spin.