Could not use file or assembly 'Oracle.ManagedDataAccess'

Question

In my application, earlier we were using entityframework of unmanaged dll of 11g Oracle.DataAccess. Now as migration work, we have decided to use managed dll of oracle (oracle.manageddataaccess) and entityframework. I added reference of managed oracle client in application and required changes in app.config as well as some other changes:

App.config changes:

<configSections>
<section name="oracle.manageddataaccess.client" type="OracleInternal.Common.ODPMSectionHandler, Oracle.ManagedDataAccess, Version=4.122.18.3, Culture=neutral, PublicKeyToken=89b483f429c47342" />

<system.data>
<DbProviderFactories>
  <remove invariant="Oracle.ManagedDataAccess.Client" />
  <add name="ODP.NET, Managed Driver" invariant="Oracle.ManagedDataAccess.Client" description="Oracle Data Provider for .NET, Managed Driver" type="Oracle.ManagedDataAccess.Client.OracleClientFactory, Oracle.ManagedDataAccess, Version=4.122.18.3, Culture=neutral, PublicKeyToken=89b483f429c47342" />
</DbProviderFactories></system.data>

After adding all these I was getting exception as Could not use file or assembly Oracle.ManagedDataAccess

I added above app.config changes in machine.config and then it worked.

My question is since I already added in app.configs then why I need to apply same changes in machine.config