You get charged with something and if you want to have the trial right now, before you have any idea what's going on, then you can insist, which basically nobody does because it's pretty crazy to go in blindActually most criminal defense attorneys recommend not waiving your speedy trial rights. Yes, the defense goes in blind. But so does the prosecution, and they're the ones that have to make a case.
The usual result for defendants that don't waive their speedy trial rights is an acquittal if the case goes to trial (between 50-60%), which doesn't sound like a lot but prosecutors are expected to win >90% of their trials. Additionally, in many counties they don't have sufficient courtrooms to handle all the criminal trials within the speedy trial timeframe, so if the trial date comes and a courtroom is not available the case is dismissed with prejudice. Nonviolent misdeameanors are the lowest priority for a courtroom (and by that I mean even family law cases have priority over nonviolent misdos in most counties), so those cases are frequently dismissed a day or two before the trial date. Consequently, most prosecutors will offer better and better plea bargains as the trial date approaches.
This is even more true for murders, which is why murder suspects don't usually get charged for a year or two after the crime.