When using oracle tapes it is easier to build hierarchies of complexity. You can, by using, e.g. some $\Sigma_1$ set as an oracle tape enumerate $\Sigma_2$ sets. Or in other words, if you can enumerate some set using some $\Sigma_n$-set as an oracle tape, you have an upper bound on its complexity.

This is just one of the things one can use oracle tapes for.