Is it necessary? Usually not.
Can it be useful? Sometimes, depending on your circumstances.
Should you use it? Basically you can think of it this way. If you're feeding a typical 15-16% protein complete layer ration to birds that are not free ranging (confined in a yard, run, tractor, etc) then at most give them half a handful of scratch feed per bird in the morning. If you want to fool with it at all. I personally do not for my tractored birds. They get their complete layer ration and pasture. Nothing more.
If you are free ranging your birds however and the ground they are foraging on is any good at all then you can feed more scratch if you like. Still not really necessary but it can train the birds as Fred says to come to you when you want them to. You should be looking over your flock every day because this is how you spot incipient problems before they become major ones. May also be useful to get them back into their yard or run if you need to lock them up before sundown. I think it best to feed scratch in the morning so that they can go to roost with a crop full of their layer feed which is when they will be building their eggs.
I use a little whole corn with my free ranged birds but no scratch at all with my tractored birds. Useful in some circumstances, not in others, not really necessary in any.