I have already researched into this and I am left slightly confused still. I have gathered that the use of a quarter wavelength is to turn a linearly polarised wave into a circularly polarised wave. But why? I appreciate any feedback.
I believe you mean "quarter wave plate" rather than quarter wave length: https://en.wikipedia.org/wiki/Waveplate
Quarter wave plates are sometimes used after polarized beam splitters (PBS). Suppose you have a PBS illuminated by a source. One polarization (say linear horizontal) goes one way and the other (linear vertical) goes the other way. Now suppose you want to split the linear vertical light a second time. If you put it through another PBS then the light won't be split; all the light will come out in the vertically polarized direction. So you insert a quarter wave plate, which turns the linear vertical light into circular, after which the second PBS splits the light in half once more.
To get a more specific answer, give us a link to a picture of the interferometer setup you are are trying to understand and we can answer what specific purpose it plays there.