re: I am trying to understand what is the purpose if these files, if they may be corrupt?
Often they are not corrupt, or the corruption may be limited to the last edit you were making, or could be isolated to just one spot of document structure elsewhere (and FM detecting inconsistent structure is probably grounds for aborting itself). The .recover, plus your .backup, usually get you back on track.
.recover is FM's best effort to save your work when it detects an error that is going to result in it crashing. It presumably uses MIF so that if the crash crashes partway through, you at least have a plainttext markup file from which to recover some work. More usually, you can recover all of your content, and most if not all of your formatting. A partially-written .fm binary file might be entirely un-openable, even with Heroic Open.
If the writes complete, you get a files that FM can open as well. If the crash crashes (or the Window, VM, or whole OS does), you might get nothing at all, as was the case for the Error 7103 crashes I used to get in a former assignment - FM would just wink out without a trace. Save early and often. Backup the entire job daily at least.
Always keep in mind that the first thing FM usually does on opening file xyzzy.fm is to over-write file xyzzy.backup.fm, so when recovering from a crash, first make copies of everything.