This is definitely one of those your mileage may vary situations. If you have a lot of originals on your device and then turn on optimizations you will see a lot of space recovered. I know when I turned it on I recovered a lot of space, several GB. Someone with only a few pictures wouldn't see much difference.
If you really want to recover space you can offload them to your computer or use a third party service to upload your phones. A lot of people post their photos on social media so do you really need to keep a copy on your phone too? Keep in mind the photos uploaded to Facebook are highly compressed once you upload or even sync them to their site. If you plan to ever print or need the originals this wouldn't be the way to go.
If you want to backup your photos I can definitely recommend Amazon Photo, Flickr or BitTorrentSync (for those will a little more technical known-how). All of these services will retain the original uncompressed images. Google Photos is also good but again they compress photos unless you turn that feature off.