No, that's where the files are actually installed to. Unless I've done something entirely bizarre, since I only have a cd6 folder in BG1. The rest of the biffs are in the data folder.
After deleting files from the data folder, do they get repopulated as you play the game?
Yes, they do; at least anyway they used to. What used to happen was that with a partial install the cache (and this is what I'm considering the BG1 Data folder to be) would fill up every so often during a playthrough and it would take forever and then some for new areas to load. Typically this would happen whenever I first entered the BG sewers. I got in the habit of emptying the Data folder when I first got to the city of BG. If I re-entered any areas that had been deleted they would be re-installed as needed from the CDs.
However, now that I've actually looked at the current Data folder, I can't recall recall there previously being any files there other than the area bifs - there are other ones there now. Regardless, it used to be the case that the entire content of the Data folder could be safely deleted.
Otherwise you're just deleting data that the game needs to read (perhaps it just checks the CDs if it can't find it on the hard drive?)
Yeah, it would get it from the CD. I don't know much (and that's an understatement) about the infinity engine or weidu or any of this stuff - and it's probably not correct to call the Data folder "cache" as I do but really, there used to be no adverse consequences for purging it whenever you wanted to do so. But I am not arguing that it should now be deleted. I just don't know much (and I'll leave it at that.)