iCloud photos used in conjunction with photo stream...

Here are other differences from this read: https://support.apple.com/en-us/HT205743

iCloud Photos

• Uses your iCloud storage.
• As long as you have enough space in iCloud, you can store as many photos and videos as you like.
• Stored in the original format at full-resolution.
• You can turn on Optimize Storage and save space on your device.
• Edits are stored in iCloud and stay up to date on your Apple devices.

My Photo Stream

• Doesn't count against your iCloud storage.
• Uploads your last 30 days and up to 1000 photos.
• Downloads to your iPhone, iPad, and iPod touch in a smaller resolution. On your Mac or PC, your photos download in full-resolution.
• Edits you make won't update across your devices.

Here is another read that says some very important things about Photo Stream: https://support.apple.com/en-us/HT201317

Using both will be making things complicated, if you are taking pictures on an iOS device (iPhone, iPad) and you are taking Live Photos. My Photo Stream is transferring the Live Photos only incomplete, only the still frame and without the videos, while iCloud Photos is transferring all items. Don't enable both on the same device. And don't enable My Photo Stream on a device, where you are taking Live Photos and videos. You will have to transfer the Live Photos and videos using a USB connection, and then the still frames transferred by My Photo Stream will prevent the transfer by USB as a Live Photo.

It can make sense if you have multiple devices and want iCloud Photos on in some of them but in others you only want photos to sync via Photo Stream.

For example, see the following from My Photo Stream - Apple Support

If you use another device with the same Apple ID that has only My Photo Stream turned on, photos that you take on that device upload only to My Photo Stream. They don't appear on any devices that have only iCloud Photos enabled.

If your iCloud Photo Library is enabled, Photos Stream is effectively disabled.

https://support.apple.com/en-ca/HT201317