Why shouldn't you break an egg directly into other ingredients?

One reason is that if you don't' do it perfectly, some of the eggshell will get into the other ingredients, and once there, can be very hard to remove.

You might also have an egg that has gone bad, and dumping it in with other ingredients before knowing is taking a risk.

If you have a small bowl available, crack the egg into that first, and then if it is okay, and free of eggshell shards, transfer the egg into the bowl with other ingredients.