iCloud Photos requires the use of Photos in iOS or macOS to sync images across all the devices with which you’re logged into the same iCloud account, as well as uploading them at full resolution to the central iCloud servers, where you can access them at iCloud.com.
But what if you have an out-of-date Mac, don’t want to convert an iPhoto library to Photos, or otherwise can’t get yourself in a position in which you can use the latest tools? For instance, some people have shifted entirely to using an iPad, but an iPad is not an effective way to upload an old iPhoto or newer Photos library to iCloud.
There’s a workaround, so long as you know someone with a Mac that you can use temporarily and they’re willing to set you up with an account. You can transfer your old Mac-based library to iCloud Photos, and then continue to use it solely via iOS and iCloud.com.