All functionality within your web app must be keyboard accessible. Functionality is defined in WCAG as "a series of user actions where each action is required in order to complete an activity" and "outcomes achievable through user action."
Keyboard accessibility ensures that users relying on keyboard navigation can interact with content and complete tasks within your web app. For example, in a login process, the form fields for username and password should be keyboard accessible, as well as any buttons, links, or other interactive elements required to complete the login process. Another example would be a "drag and drop" component, which should be moveable using the keyboard alone.
The only exception to this criterion is when, as described in WCAG, "the underlying function requires input that depends on the path of the user's movement and not just the endpoints." A common example of this type of function is a drawing activity, where the user's full movement path is used to create the drawing.
A keyboard navigation helper can assist with visualizing keyboard navigation in your web app, and is one of multiple helpers included in the Guided Manual WCAG Audit Tool from Accessible Web.