The point is to try to avoid putting yourself in a position where you have to throw someone into the fire who isn't yet ready for it (see Buchholz last year).
If you throw a kid into the fire and he sucks, all you have to fall back on is maybe another kid -- if you're lucky. If you sign a veteran instead and he sucks, you have a chance of promoting a kid to take his place -- and have time to figure out exactly which kid it should be. It's an extra layer of insurance against getting absolutely no production at ALL from a spot in the lineup and a chance to buy time to allow merit to sort out who deserves to be getting chances based on what they're doing (AND how they're doing it!) in the minors.
If we hadn't signed Penny, Saito and Smoltz, then we would have broken camp with Buchholz or Bowden in the rotation and very little minor league insurance at SP. Daisuke's injury would have been a potential disaster, coinciding as it did with Buchholz' hammy pull. Bard would also be in the bullpen right now occupying Saito's space, ready or not. Bowden would be in service now either as a SP or an RP. That's a lot of pieces with a reasonable chance of failure and not much behind them but AAAA scrubs and reclamation projects, besides whatever damage a premature callup might do to the development of the kids we had on the roster.
Put it all together, and a couple decent filler types with upside are a good deal to make in the offseason for a team in player developpment phase, even if they don't exactly play up to their ability while on the roster they tend to have some use just because of the guys who aren't there.