Actions have a "description" information that clarifies what they do. When actions are displayed inside the "more" ("...") menu, there is room to display such clarifications in plain text (T126738). However for secondary actions displayed directly on the notification these descriptions can be displayed as tooltips.
In some cases the tooltip is used to display the full name of the action because it is truncated. In such cases, displaying the full action title in the tooltip is still preferred.