The answer is that while it does not change the raw numbers of each party, it can and does change the number of representatives in Congress for each party. Let me explain.
Let's say for the sake of argument that 50% of the people are Republican and 50% are Democrat in your state, and you have 6 districts. You would assume, all things being equal, that half your representatives (3) would be Republican and half (3) Democrat, insuring that all the state's citizens are represented fairly. You would be very, very wrong in making such an assumption.
Over the last 60 years the majority of Governors during redistricting years were Democrats, so they got to do the redistricting. Here is what would then happen in the 50/50 state we illustrated earlier, assuming there were 6 districts in the state...
The Democrats would first determine which communities - and even which neighborhoods - are primarily Democrat or Republican by reviewing voter registration and census counts. They would then draw a district that is designed to include 60% Democrats and 40% Republicans, insuring that a Democrat would be elected in that district. They would do that for 5 of the 6 districts. The final district would be made up of primarily republicans.
And now, in a state where Democrats and Republicans are equal in number you have 5 Democrat representatives and 1 Republican representative.
Not very fair or equal, is it? When one party can, by design and intent negate the votes of 40% of the people you end up with a government that does not represent the people.
That is also why we have "career" representatives who, no matter how bad they may be, can remain in office for life.
Take one more look at some gerrymandered districts, and you begin to see just how crooked the system really is, and why this election year for governors is of great importance for Republicans, since the vast majority of all districts nationwide have been drawn by Democrats. Republicans need to try and equal it out a bit. If they do not, we are guaranteed another 10 years of Congressmen who do not represent America and refuse to listen to their constituents - because they do not have to.