Let's say there is plenty of convincing evidence that A killed B. In the USA, this could be first degree murder, second degree murder, manslaughter. Now let's say there is some evidence, but not necessarily enough for conviction, that it was first degree murder, and more evidence that it was second degree murder.
In that situation, A could be charged with first degree murder, second degree murder, and manslaughter. A would then be found guilty of the highest charge that is "proven beyond reasonable doubt".
Compare this to charging only with first degree murder: That would mean A must be either convicted of first degree murder, or set free. Now if the jury thinks there is plenty of evidence for second degree murder, but not enough to convict of first degree murder, they have the awful choice of setting a murderer free, or to convict him of something he is not guilty of. That's a situation you would want to avoid.