FrameworkElement and FrameworkContentElement include an implementation of OnPropertyChanged that monitors effective value changes to all dependency properties that exist on an element. As part of that logic, dependency properties that change effective value and have metadata with AffectsRender set to true will initiate a deferred request to invalidate the visuals for that element. Because this WPF framework-level implementation is already in place, you typically do not need to look for dependency properties with AffectsRender unless you are substantially replacing or modifying the WPF framework-level layout behavior.
Custom OnPropertyChanged implementations might choose to have similar behavior for dependency property changes where AffectsRender is true.
Properties on any derived classes of PropertyMetadata are typically defined in the object model as read-write. This is so they can be adjusted after initialization of the instance. However, after the metadata is consumed as part of a call to Register, AddOwner, or OverrideMetadata, the property system will seal that metadata instance and properties that convey the specifics of the metadata are now considered immutable. Attempting to set this property after IsSealed is true on this metadata instance will raise an exception.