1. This features that you need to attribute because they are the owner of the songs who made and you need to to make them credited as you used their music.
2. Without informing them that you are using their music you can't monetize your videos to earn money
3. Your video would be blocked or it will not run over public viewers on YouTube.com
4. This is important now, because - imagine how you feel if someone stole your own created music without attributes, anyway, they are really selling their music on YouTube that's why they let them do apparently like you do with business shopping but free which is already set as for public using.
5. Attributing their music meaning you have the respect and discipline, a way to show you are thankful and you are giving back the good provision.