The Apple's iMessage chat and back again. This makes it easy to choose the best format to continue a conversation at any given time. For example, when there's a need to be quiet, switching to an iMessage allows the conversation to carry on without interrupting anyone else's sound by talking during a FaceTime call.
Apple's FaceTime features have expanded greatly over the last few years, bridging the gap in face-to-face conversation that came along with the global pandemic. An email is a nice long-format way to share information. Messages provide instant reactions for an even more satisfying chat. However, nothing can really compete with the easy expression and nonverbal communication that is possible with FaceTime. So there's really a need for all three to handle every situation.
It's even possible to use FaceTime with picture-in-picture while typing in Messages. From an iMessage, the can tap the FaceTime button in the upper-right that looks like a video camera. A menu appears with an option for a video or an audio call. Tapping either sends a request to the other person in the chat that appears as a button at the top-right. Tapping opens FaceTime and connects the call. The iMessage isn't closed but is simply hidden by the FaceTime app.
Switch From FaceTime To Message
doesn't affect the iMessage, allowing for an easy transition.
Apple doesn't restrict these features to the iPhone. An iPad and Mac have the same capabilities to move from FaceTime video or audio calls to iMessage chats and back again just as easily. On a Mac computer, Messages and FaceTime can run side by side or be maximized to fill the screen without the need for buttons to switch between the two. With an iPad, the extra screen space makes PiP more useful, and Apple used the buttons as on an iPhone but changed the layout a bit. The iPhone is still the most convenient for FaceTime since it's the most portable and has the best selfie camera.