mapi32.dll is the core MAPI stub library you should be loading, unless you know you only wish to work with Outlook's MAPI (or the MAPI of some other product). Because we didn't always used to have a MAPI stub library, some products still try to replace mapi32.dll with their own implementation of mapi32.dll. So, we also keep a copy of this library as mapistub.dll. That's why, if we find we can't work with mapi32.dll, we try mapistub.dll instead, in case mapi32.dll has been clobbered.
As for why you'd want to get the underlying MAPI implementation library, for most basic MAPI operations, you're right, mapi32.dll is sufficient. But Outlook's MAPI does implement some exports that aren't part of the core MAPI specification. So you might want to find Outlook's library so you can load exports from it. Other MAPI clients may provide their own custom exports you also wish to code against.
As for why you'd want to get the underlying MAPI implementation library, for most basic MAPI operations, you're right, mapi32.dll is sufficient. But Outlook's MAPI does implement some exports that aren't part of the core MAPI specification. So you might want to find Outlook's library so you can load exports from it. Other MAPI clients may provide their own custom exports you also wish to code against.