People always try to learn new things to improve their knowledge. Be it a language or a new skill, we will gain advantage from the acquired new skill. To learn a new skill takes a long time. But time is a limited resource. It is not necessary that you should devote all your time to learning a new skill. Instead, we should spend the time productively.
You can compare your brain with a bucket. Your brain can be either like a bucket without holes or the one with holes. Either your brain can store and retain all we learn or drain out all we have learned. As said earlier, we can't increase the time we learn. But can increase the capacity of our brain.
It is quite common for us to forget certain things which we captured through our brains. Certainly, this should not be construed as a negative statement. This is how our brain has been designed. Everyone is not gifted with IQ capable of grasping everything. It is quite interesting to know that our brain can absorb only as low as 10% from the lectures our professors give in our class and 20% of the reading you do from the books.