Question: In some appratus for the Young's double slit experiment a collimating lens is used infront of the double slits, meaning plane wavefronts hit the double slits. But why do we need/want plane wavefronts hitting the slit in the first place, won't spherical wavefronts work just as well? Also if we did not have plane wavefronts would we still get Fraunhofer diffraction (I assume we will since we will still be viewing the image in the image plane).

Answer: 
It depends on the coherence length of the light source. If the wavefronts are curved, and it's difficult to ensure that light generated at the same time is impinging on the slits at the same time -- you need a relatively precise alignment in both the centre of the lens and the angle.

If you collimate a beam, the wavefronts are planar, so it's less sensitive to a movement of the centre of the lens.
It depends on the coherence length of the light source. If the wavefronts are curved, and it’s difficult to ensure that light generated at the same time is impinging on the slits at the same time — you need a relatively precise alignment in both the centre of the lens and the angle.
If you collimate a beam, the wavefronts are planar, so it’s less sensitive to a movement of the centre of the lens.