not all transfers are done in installments - Vidal's and Rossi's potential ones are/would likely paid up front.
This sort of business is popular among Italian clubs, and in other leagues too, and is helpful to a team with a limited budget to still operate on the market.
Of course you have to pay the installments due in a given year but, again, it allows a team to buy several players in a season and minimize that particular transfers' burden on the budget for that particular year. (this is also a sort of insurance against one player not fitting well in a team or getting a long-term-injury)
Milan for one, couldn't afford a one-time 24 mil fee for Ibra last season, but spreading it around over 3 years, made the deal possible. Of course, 8 mil would come out of the budget in every of the next 3 years but that allowed Milan to also afford Robinho last year too as there was no chance in hell they could pay 42 mil, all at once, for both even if that was a bargain price.
installment deals made it possible for Juve to buy several players last year when you clearly couldn't afford paying for them up front. Juve's problem is not that you guys made deals in installments but that you ended up buying players like Motta, Pepe, etc ... now imagine if those deals had been for Hernanes (as Lazio did), Asamoah and Cavani. Wouldn't you just love paying small amounts for those players even if it's for the next 3 years?