I believe that, (possibly only) in the early days, it offered to add any imported music to your library, which I think it indeed did by converting to AAC and putting in a directory structure of its choosing.
It probably could convert to AAC, but the biggest issue for collector nerds with carefully curated collections of mp3's organized in folders by artist and album and with perfect metadata was that iTunes would take all your hard work, shake it around, rename all the files and dump them in a single folder and fuck up your metadata. At least that's how I remember it, but it's over 20 years ago ...
As I recall it, the AAC option was for syncing to iPods with lower capacity (like the Shuffle) so that it would convert your 320/256kbps MP3s to 128kbps AACs to fit more music. It did this for a copy of them though, I've never heard of it actually converting your main library and checking out an old version of iTunes it has no way of converting your whole library (just individual tracks one by one).
Having iTunes organize your music into its own folder structure was an preference you could turn off, but that option was not shown in the onboarding so it ruined many a people's meticulously organized files...