What is the reason for using a "Lines-Oracle" in the PCP Theorem?
The lines oracle is used to decrease the query complexity of the test from $d+1$ to $2$, at the expense of using a larger alphabet.

If you don't mind making $d+1$ queries, then the lines oracle is indeed unnecessary. However, it is usually better to make two queries over a large alphabet than $d+1$ queries over a small alphabet.

One reason is that, very roughly speaking, we have a technique, called PCP composition, which can be used to reduce the alphabet size of PCPs, but cannot be used to reduce the query complexity. Hence, it is preferable to have small query complexity and large alphabet size rather than the other way around.

A second reason, which is very related to the first one, is that many hardness-of-approximation results are proved based on $2$-query PCPs, but can not be proved using based on a PCP with more queries.

It should be mentioned that there are also generic techniques for reducing the query complexity at the expense of the alphabet size, so one could avoid the lines oracle and apply those techniques instead. However, using the lines oracle gives a more direct proof.