To put it simply -- a blind person believes there is a Sun either by trusting secondhand accounts or by deducing / inferring the fact from other available (sensory) data. Yet what Sun actually is, even if its presence is correctly assumed or believed to the point of complete certainty, remains in the realm of imagination. A person who can see, on the other hand, knows exactly what is meant by Sun and there is no doubt.

Of course that is a rather crude analogy because in this example even the person who can see has yet to realize that Sun is a ball of burning gas, etc. and not e.g. some lamp hanging in the sky.

Now for why it makes a difference -- imagine the blind person was told (or an idea occurred to them) that Sun is a heater that is always out of reach and goes off for half a day but one has to perform certain actions to make it turn on again. This might lead to behavior that could range from absurd to outright dangerous.

Conversely, the person who can see is free of it all as they can take the Sun at face value -- what they see is what they get (thus, me using "beyond any doubt" in relation to knowing was probably totally redundant, then again, sometimes one initially cannot believe one's own eyes ;P).

In this way knowledge of God (as opposed to belief or nonbelief) removes all misconceptions that can have disastrous consequences at worst or lead to behavior that is sort of levelheaded and "moral", nevertheless still upholds a society as we know it today, at best (and it's obvious that our societies have a lot to be desired).