Unity ships with multiple Standard Assets packages. These are collections of assets that are widely used by most Unity customers. When you create a new project from the Project Wizard you can optionally include these asset collections. These assets are copied from the Unity install folder into your new project. This means that if you upgrade Unity to a new version you will not get the new version of these assets and so upgrading them is needed. Also, consider that a newer version of e.g. an effect might behave differently for performance or quality reasons and thus requires retweaking of parameters. It’s important to consider this before upgrading if you don’t want your game to suddenly look or behave differently. Check with the package contents and Unity’s release notes.
Standard Assets contain useful things like a first person controller, skyboxes, lens flares, Water prefabs, Image Effects and so on.
Sometimes you might want to upgrade your Standard Assets, for example because a new version of Unity ships with new Standard Assets:
Note that the upgrade will replace files for which a newer version is available but it won’t remove obsolete files from the previous version of the Standard Assets. To avoid such unwanted files, you might want to remove the old Standard Assets from the project before installing the new version. However, it is wise to keep a backup copy of the old assets just in case your project depends on them in a way you didn’t anticipate.