If cache is 100% full and I don’t clear the cache, what will happen?

depends on the amount of disk space you have at your hoster, but there are other more important disadvantages you should take in consideration, so it would be better to reconfigure AO (disabling “aggregate inline JS” and “aggregate inline CSS”) to ensure the cache doesn’t grow that quickly.

hope this clarifies,
frank

so meaning to say that don’t let the optimiser to reach 100% cached. so that we don’t need to clear the manually. Am I right?

absolutely, yes.