Until industrial strength became the criterion for economic success, which was just starting to happen in the 18th century, Italy was extremely wealthy due to its leadership in trade and commerce. From Renaissance times it was a target for stronger nations such as France and Spain to try for a little empire-building, largely because of it's fragmented states which were not always strong enough to deflect national armies, but at any rate were usually good for a bit of 'smash-and-grab' forays. By the 18th century, the major nations took the more civilised route of marrying heiresses, and extending influence rather than sending the army in, but Italy was still potentially very lucrative. We tend to think of Italy in terms of a less industrialised nation compared to those of northern Europe, but it wasn't necessarily considered so at this period. Austria until Maria Theresa's time had been extending its empire into Bohemia and Hungary and was concerned to ensure the Turks did not encroach, but were now starting to look southward - and they were closer than France and Spain. Parma certainly didn't offer military advantages in itself, as you say Ivanushka, but it certainly wasn't poor in 18th century terms, even if it wasn't as rich as some of the Italian states. And once established as a sort of patron state, a stronger nation would be in a really good position to do a bit of territorial aquisition elsewhere in Italy, using Parma as a base, if the opportunity was there. So it was well worth the while of France, Spain and Austria to jostle each other a little, in a very civilised way, to keep Ferdinand under their thumb - though in the end, none of them managed it, at least not through Maria Amalia.