In the case of the 2X it multiplies the focal length by 2 times, with the loss of two f stops. The 1.4X multiplies focal length by 1.4 X while losing only one f stop of lens speed. This means that your Carl Zeiss T* Sonnar 180mm f2.8 will be an effective 252mm in focal length at f4. That is an appreciable gain in focal length without a prohibitive loss in lens speed.
When the effective f stop changes by the introduction of a tele-extender or with bellows extension, the exposure changes but the depth of field stays the same as the original lens f stop.