Let's look at two sentences:
1) I'm sewing a flag for the party.
2) I will be sewing a flag for the party.
Sentence #1 can mean that I'm doing it now OR that is my plan to do so (probably in the near future).
Sentence #2 can mean that I plan on sewing the flag, but in the future. Or, if paired with an explanatory sentence like, "Sorry, I can't make it to lunch tomorrow," that it will be an action I will be busy with at the same time as another future action.