This video explains why most of our genome must be non-functional.
Let's see if I can summarise it.
If most of our genome is functional and even a tiny number of mutations occur and some of those are deleterious, then we must have many offspring to maintain a population.
If 100% of our genome is functional and necessary (the two words seem to be equivalent in this situation) and there is a mutation, it is around even odds (I am paraphrasing and simplifying) that it will be neutral or bad. If it is bad, the offspring is likely to die. So much so that a couple would need to have many children - hundreds- so that two would make it to reproduce to create the next generation and maintain a stable population.
If the genome is less than 10% functional, most or all mutations will be neutral - changing a non-functional section means there is likely no change to the organism. Parents in this scenario need to have around 2.1 offspring who reach adulthood to maintain a stable population.