Since either side can file such an appeal, how does bias toward the prosecution do that?
Not either side can file an appeal. Well, I guess they can file them, but defense (interlocutory) appeals will be dismissed where state (interlocutory) appeals will be heard.
The state has an advantage in having its interlocutory appeals heard, because it only gets one bite at the apple. If the trial court gives a reversible advantage to the defendant, and the defendant is acquitted, the state is done, fini. Double Jeopardy attaches. So, the appeals court will hear state appeals before and during the trial, on matters where it would not take up exactly the same argument if it was presented by the defense.
The appeals court can, if defendant is convicted on an error, vacate the conviction and remand the case to the trial court for a new trial.
Convictions can be reversed on appeal. Acquittals cannot.