Hi,
You will make detail objects when you have unbalanced hierarchy in your different data providers.
Say for e.g. DP1 has D1, D2,D3,M1 and your DP2 has D1,D2,M2
Now you see that you have D1 and D2 common among DP1 and DP2 but in order to incorporate D3 in your block where you want to see all the data from DP1 and DP2.
you need to create a Detail object for D3 based on merged dimension so that it gets sync with your both data providers. If you directly use D3 in that block it will show you #DATASYNC error. so this Detail Object solves your issue in that case.
Let me know if I was able to explain you.
Regards
Niraj