While I agree in principle, it does look like in most cases, these 100X large genomes are just due to random events that haven't been selected away. Often, what you see looking at evolutionary processes at a point in time, you want to find a "reason for why it's this way", when the reason is "it was random chance". Compare deinococcus radiodurans, where there's a whole mechanism for exploiting multiple copies of the genome. Plants wiht 10X genomes don't seem to exploit that, afaict.

It’s always random chance. What’s non-random is why it’s kept in the genome. I’m no plant expert, but genome duplications could be linked to increased fitness due to amount of resulting protein product, right?

If it didn't give an advantage it shouldn't have become dominant (that all individuals of that species have that) or not?

that's now how evolution works. evolution tolerates enormous amounts of things that don't 'give advantage', and 'advantage' is a fairly subjective term anyway. Any number of possible events could have occurred to lead to the scenario we see. Population bottlenecks often lead to strange outcomes.