Because in your research, you are likely to come across several citation styles, and one style needs to be chosen and all cites reformatted
To the right, you will see four references to the same article from the scientific journal Nature. They were copied and pasted, with formatting included, from four other journal articles, from four different journals. If you find an article that cites an article that you are going to cite, and you copy and paste the citation into your paper, and you repeat this ad nauseam with all of your myriad references, your bibliography (or list of Sources or References), will on its face appear inconsistent and unprofessional.
Notice, some of the various differences in the four styles:
Thompson, L. R., Sanders, J. G., Mcdonald, D., Amir, A., Ladau, J., Locey, K. J., et al. (2017). A communal catalogue reveals earth's multiscale microbial diversity. Nature 551, 457. doi: 10.1038/nature24621
Thompson LR, Sanders JG, McDonald D, Amir A, Ladau J, Locey KJ, Prill RJ, Tripathi A, Gibbons SM, Ackermann G, Navas-Molina JA, Janssen S, Kopylova E, VázquezBaeza Y, González A, Morton JT, Mirarab S, Zech Xu Z, Jiang L, Haroon MF, Kanbar J, Zhu Q, Jin Song S, Kosciolek T, Bokulich NA, Lefler J, Brislawn CJ, Humphrey G, Owens SM, Hampton-Marcell J, Berg-Lyons D, McKenzie V, Fierer N, Fuhrman JA, Clauset A, Stevens RL, Shade A, Pollard KS, Goodwin KD, Jansson JK, Gilbert JA, Knight R, Earth Microbiome Project Consortium. 2017. A communal catalogue reveals Earth’s multiscale microbial diversity. Nature 551:457–463 DOI 10.1038/nature24621
Thompson, L. R. et al. A communal catalogue reveals Earth’s multiscale microbial diversity. Nature 551, 457–463, https://doi.org/10.1038/nature24621 (2017).
L.R. Thompson, J.G. Sanders, D. McDonald, A. Amir, J. Ladau, K.J. Locey, R.J. Prill, A. Tripathi, S.M. Gibbons, G. Ackermann, J.A. Navas-Molina, S. Janssen, E. Kopylova, Y. Vazquez-Baeza, A. Gonzalez, J.T. Morton, S. Mirarab, Z. Zech Xu, L. Jiang, M.F. Haroon, J. Kanbar, Q. Zhu, S. Jin Song, T. Kosciolek, N.A. Bokulich, J. Lefler, C.J. Brislawn, G. Humphrey, S.M. Owens, J. Hampton-Marcell, D. Berg-Lyons, V. McKenzie, N. Fierer, J.A. Fuhrman, A. Clauset, R.L. Stevens, A. Shade, K.S. Pollard, K.D. Goodwin, J.K. Jansson, J.A. Gilbert, R. Knight, The Earth Microbiome Consortium, A communal catalogue reveals Earth’s multiscale microbial diversity, Nature, 551 (2017), pp. 457-463, 10.1038/nature24621