If definitions are clear and everyone agrees on them, then discussion can be about whether those definitions apply. Discussion is then about the real world. The definition of murder is pretty clear and widely agreed on; there is plenty of room for discussion about whether it was Smith or Jones who committed it.
If definitions are unclear or people disagree on them, then discussion may either be about the definitions and not about the real world, or be incoherent.
The activity of unpacking those words is important, because in the process you get to an actual debate and not just people talking past each other. Once you do it everyone involved realizes that they mostly agree anyway, that they actually disagree on something fundamental, or that some people are intentionally clouding the argument to try to make a dishonest point.
The real problem is when you and another person have a different working definition of something "everyone is clear and agrees on" and make plans or agreements around the assumption of shared definition. The way you will find out will not be fun.
Anything else, that everybody is clear and agrees on, might as well not be discussed at all...