Why are there different Newman projections for one molecule in organic chemistry?

source (for the diagram) here

Since one can rotate the C-C bonds about each other we can have a number of conformations.

conformers or rotamers are the isomers obtained when a molecule is rotated between a selected C-C bond
.In the diagram shown above a propane molecule
`C_aH_3-C_bH_2-C_cH_3` where a,b,c are the labels assigned.
if one rotates this about `60^@` molecule about any of the `C_a-C_b or C_b-C_c`
we obtain the different rotamers as shown in the diagram

If one looks at the different rotamers shown:
The first one consists of all the atoms in an "eclipsed conformation".Since these atoms are close to each other they repel and the magnitude of repulsion depends on how bulky or big the substituent is.
Due to the presence of bulky groups and the repulsions caused the energy of the molecule in such conformations are highest while compared to the other conformations.
Similarly we have staggered conformation in which the bulky groups are far away in such a way that the molecule's energy is least and hence it is stable.
.
example:
Proteins are macromolecules.We define dihedral angle ie. the angle between two planes. At a particular conformation the proteins exhibhit catalytic activity.