My immediate answer is it can make them sick. Bread, cheese and corn is not what geese should be eating. It can get moldy in the water and it makes them poop more which isn't a good thing for geese.
But I thought I'd do a bit more research so I could more fully answer his questions.
I see little children feeding geese and I know that most of the time it's just fine, but there are times when it really isn't fine. Geese can hurt a grown up, and very seriously hurt a child. Feeding geese develops a lack of respect in the geese for people and a lack of respect in people for the goose itself. You know, the whole familiarity breeds contempt. Geese should walk away from you when you pass by, but geese that are fed by humans often get pushy and bossy and that's when they are prone to becoming dangerous to people. A dangerous goose is not one that will live long eh?
BUT not only does it do that. Feeding geese can actually HARM them.
Bread and corn and pastries and crackers are NOT good nutritional food for geese. They thrive on grasses and underwater plants. As herbivores that is what geese NEED to eat. They don't need to eat items of low nutritional value as a bulk of their diet....they need plants and lots of them. Remember.... it's not just YOU feeding the geese, but it's all the other people feeding them as well. If you want to feed them... feed them plants and grasses and things that are actually good for them.
When you feed them on shore you are also feeding them where they defecate... not such a good thing eh? Think on that for a bit. And think for a moment about where the poop goes...on the grass, on the side walks and all over the place (you want to walk in goose poop?), and eventually washed into the river/lake where it causes algae bloom.
By feeding them you encourage too many geese to stay in one area. Some parks are so crowded with geese it's difficult to find a place that is away from them. Overcrowding leads to more disease and makes it easier for disease to spread. Overcrowding can also lead to pests.... geese are a prey animal. Concentrate a prey animal anywhere and guess what moves in...predators.
The young learn that people mean food. What do you think that means when they migrate? Geese are often hunted. A tamed or used to people goose is much easier to hunt as they have no reason to fear us.
Young geese end up being fed wrong and develop health problems.
You can delay their migration as it's easy food, why leave? That means when they do migrate you have just made it more difficult for the goose to survive its journey south.
You know, I recognize that it can be a lot of fun to feed the geese, I know that it delights children to feed them, but seriously.... it's not a good thing.
If you want to feed a goose....go to a petting zoo or find some land and keep a few geese of your own. But don't feed the wild ones...it's not good for them and long term it's not good for you. :)