Lolix already said the most important thing here but just as a side note: Don't worry too much about challenge missions, sure they are a decent way to get both fairies and affection items every day of the week but they are not that important unless you desperately need said resourses; BUT every few challenges will unlock 1 extra spot on your main barrac (for units) so if are running low on space that's a way to get a few extra spots