? They are both independent "base games". For example there are no separate DLCs for the GOTY version, but there are many DLCs for this normal version. It's totally normal in this case that if you own the GOTY version, you can activate this normal version as well or vice versa. This does not apply to all Steam games that have GOTY version out: for example Borderlands 2 does not behave like this.

I didn't mean the value in that sense, but in this way:
-From game collector's point of view, this version of this game is increasingly valuable to obtain, because sooner or later the available copies of this game run out, or at least become rarer to find
-One can actually sell the steam account itself. Although it's also against rules but still happens. Having this specific version of this game on the account may add significant value to the desirability of the steam account depending on the buyer of course