No sound should end suddenly in a composition, and no sound should start suddenly. Why do you think is that? Imagine that a powerful sound would start suddenly. Most likely you would be frightened, or even worse.
Let's say that you don't get scarred, but still, music is not something brutal or unpleasant. Quite the opposite.
The conclusion is that the reverb does the same thing: it fills the empty spaces between the sounds that have transients so they don't end instantly. Even more, it increases the duration of the sound.
As I was saying earlier, a sound that ends fast gives a unpleasant sensation. Even in the aggressive styles of music no sound starts or ends fast. Why?
Because if the volume is very high and in starts suddenly it would be unpleasant. The human ear must be prepared for such volume.
The reverb is an effect of refinement and design. It's extremely important in the creation of a pleasant ambience. The only thing is that it should be used properly.
Now lets talk about how to use the reverb. As I was saying earlier: use more reverb on short sounds with transients, and use less on long sounds(like pads) which already are 'rich'.
Is not a general rule, you should set it by the context in which you use it.
The reverb effect should be inserted on a separate channel. The dry sound should be set on a channel, and the wet sound should be set on another channel in the mixer.
I do not posses knowledge regarding other DAWs, but in FL Studio you can use sends. FL Studio is quite limited, it's mixer having only 4 sends.
So if you have more than 4 instruments in the mix and you wish to add reverb to each one of them then you can't add reverb for all.