The idea for the Rect Transforms started back in Unity 4.3 timescales with the introduction of the new Sprite system, they were exposed through the sprite editor when you were editing spritesheets, where you had a rectangular area to identify the area on the spritesheet where your sprite texture would come from. This was then used as the basis for drawing all sprites, you just didn't get to see it.
Seeing the need for more control over how sprites were drawn in the scene gave birth to a new manipulation control in the Unity editor called the Rect Tool, which took its rightful place in the main toolbar as shown here:
In its default mode, this tool allows visual scaling of a sprite as you can see here:
Tip
If you cannot see the blue control points, you likely need to scroll the view in. At low zoom levels the controls are not displayed around sprites/objects.
You get the four control points (one in each corner) to scale the sprite in or out from that point and the...