Why to add the DLL's under the Solution Folder? why not under BIN?

Hello,
I am new to this solution folder concept as Till now I have been working on small sites.
In the new company I have joined. They have an application where Under a solution file they have a Solution Folder called Lib and all DLL's are under this folder. Also there is a Debug and Release Folders under this Lib Folder which also has the same DLL's.
My Questions:
1. Why all the DLL's are under this Lib folder. Why not to put them in the BIN folder?
2. If we have Debug and Release Folders with all the DLL's under this Lib folder then why do we have the DLL's directly under the Lib folder. Why to have them at 3 places. ?





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