There are many aspects of judo training that are important but the single most important thing is randori.

Of course technical training is important, so is gripping training, drills, fitness and so on. But nothing is comparable to randori for fighting skills development.

To illustrate how important randori is, I would give this extreme example. Let's say I have a bunch of kids who are fit and athletic but are absolutely beginners. Without teaching them a single technique, I ask them to pair up and wrestle. "Even though you have no technique, just do what comes instinctively... but don't grab legs!"

If they were to do an hour of randori like that for a year, without any instruction, at the end of the year, they would have become pretty good fighters through trial and error alone. If you throw in a few senior players to randori with them, the beginners would improve further (without any intruction) through osmosis.

In the course of a year of doing this much randori (daily, for an hour each time), they would become good fighters. Would they have finesse? No. Would they know a wide range of techniques? No. But they would know how to fight.

When you "play" judo that much, you can't help but improve.

Imagine taking a bunch of kids to a field and asking them to play football every day for for one hour, for a whole year. Even without any coaching, is there any doubt they would be good at the game after a year of this? And, of course, if you throw into the mix a few experienced football players, those beginners would become even better.

Back to judo. Now, if in addition to randori, you also throw in technical instruction, gripping strategy and situational drilling, imagine just how good they could become.

But if you take randori out of the mix, they won't be very good at fighting even if they have all the technical knowledge in the world. Drills will help but there's only so much you can achieve with drills. Randori is both valuable and important.