Google's Photos application makes storing and saving your memories easier with each new iteration, but storing images in the cloud instead of on device does carry possible limitations based on how you use it. Modern smartphones take photos in very high detail, with some of them capable of shooting in RAW format, and resulting pics can end up being very large in terms of file size. Since most users opt for the free version of the company's online backup option, the shots are saved at a lower quality, but choosing the original image option means space can fill up really quickly. So, by default, Photos stores images with the more space-conscientious "high-quality" option, rather than saving an uncompressed original copy. While the differences aren't necessarily noticeable immediately or at least without taking a very close look at images saved in that way under most circumstances, that isn't always the case.
To begin with, Google's compression method is great. The majority of Smartphone cameras on the market as of this writing don't support file formats or the level of detail that would make a perceptible difference between the two storage options. That isn't true of all smartphones, however, since at least a few flagship devices boast either higher resolution cameras, larger apertures, the capability of shooting in truly lossless RAW format, or some combination of those. Photos shot in RAW format are affected to a much greater extent than other formats because of how RAW works. With those images, all of the sensor data is saved instead of just what is immediately visible on the screen. That includes various levels of light and detail which can be used later in photo editing. Those details are lost when saving via the high-quality option in Google Photos because they are automatically converted to jpg format. RAW shots are also much larger than a typical photo, so available storage will quickly fill up if those are saved in original format. For other images, the differences are such that most people will never even notice the difference, but that can create problems too for users who don't always pick the same camera settings for every situation and the difference is easily missed in almost every case. Users could end up losing detail for shots they intended to save at original size if they don't adjust accordingly for backing the photos up.
Google has its own recommendations and guidelines for how and when the different storage options should be chosen. The high-quality option allows for unlimited storage. Since it won't save any captures shot at larger than 16-megapixels, all images larger than that will be scaled back to match that. Videos are affected too and anything shot at more than 1080 HD will be scaled back to 1080p. On the other hand, backing up images as originals limits storage space available - without paying for anything - to 15 GB. Google recommends the original quality option for photos shot at more than the aforementioned megapixel setting and for videos shot above 1080p. It bears repeating that most saved photos won't be noticeably different under most circumstances and you can browse through the images below to check for yourself whether you can make out any differences.