I mean, it isn't meaningless, just culturally subjective and lacking a rigerous definition. Berries are a set of specific fruit, which fruit being included being determined by the culture in question base on percieved similarities and historic uses. We use it to quickly bring up the specific group and whatever vague characteristics we percieve them to share.

So, the definition for berries that you seek is simply "the fruit people you're interested in would point at and identify as a berry", which is a vague definition and not rigerous at all, but most people would in fact think of the same thing you do if you say "I put berries on top of my cake". If I ask my wife "hey, on your way home swing by the store and buy some berries, any type will do", she will not bring a watermelon. She in fact will buy what we both agree are berries, and so the word has useful meaning.

You'll find most classifications humans have do this too. The real world is really good at refusing to fit into the neat boxes we made to classify it and the things in it, and yet we can still use them fine enough as long as we don't get lost in semantics and wondering if a hot dog is a sandwich or cereal soup.