As it is in real life - some house keeping doesn't hurt nobody! If your hallway is loaded with lots of messy stuff, you just can't walk through it as fast as you want to.
And it's the same with Docker. Cleaning and removing unused and dangling images and volumes makes things faster again.
Removing all unused images can save Gigabytes of space on your machine.
ON HOSTdocker image prune
Removing volumes that are not used by a container anymore keeps overhead small...
ON HOSTdocker volume prune
For further information, please visit the Docker website