"I eat sweet bread."
It can't be used as a replacement. It is a topic marker. In this sentence using "wa" you would imply that the sweet bread eats by saying "As for the sweet bread, it eats". -- this particle can be used in the place of "ga" sometimes because the subject can also be the topic, however the focus of the sentence is changed. For example, Watashi wa john desu (I am john), Watashi ga john ( I am john). In the second sentence there is a Stress on "I" as if you are with your twin brother and someone talks to him as if he is you, and you need to make it clear that you are yourself. -- I am also learning so feel free to correct me but i hope it helps :)
食べます is a transitive verb, aka an "action verb." When you use a verb like this, you need to mark the word that is "receiving" the action with a を. So in this case, the thing that is being eaten ("receiving" the action of being eaten) is the sweet bread, so it needs a を. You'd do the same thing with a verb like のみます (to drink) or 見ます (to watch).