Some cases have to be tried in front of a judge or jury. Some cases both sides agree that the State of Texas would clearly be able to prove the accused was guilty, but they cannot come to an agreement on punishment. So, many times in those cases an open plea or slow plea happens where the defendant can plead guilty to the offense and present evidence to a judge or jury to request a lower sentence than the State's recommendation.
A good example of an open plea would be a case where the defendant is charged with Aggravated Assault
and the State and defense think it is a bad case for trial and they both think the defendant should be on probation. The State will not offer anything but regular probation, and the defendant wants deferred adjudication
to avoid a criminal conviction. That would be an opportunity to do an open plea and ask the judge for deferred.
However, sometimes the defense and State both want to resolve the case prior to trial and they are able to negotiate a reasonable plea offer on sentencing that both sides feel they can live with. Then, they enter into a plea bargain agreement and ask the court to accept it.