Hedging their bets
Garden birds raise multiple broods as a very sensible insurance policy. If you lose that first set of fledglings to a predator, you haven't blown everything on one effort. The chance of nest visits being noticed by a predator also goes up proportionally with the number of eggs in a clutch: thus smaller can mean safer.
Ready to fly
Unless it's obviously injured, resist the temptation to interfere if you see a baby bird hopping around on its own. It's unlikely to have been abandoned. The chances are that its parents are nearby, keeping an eye out for their offspring or looking for food. Give the fledgling some space while its flight feathers mature - it'll be on the wing in a couple of days.
It seems remarkable, but young birds don't normally have the help of an adult bird when the time comes for their first migration. For many species the long-distance trip - sometimes thousands of miles - is made using their own hard-wired instincts, without any parental assistance.
Before gaining their adult plumage, juvenile blackbirds are often mistaken for thrushes on account of their speckled red/brown feathers. Moulting - where older feathers are replaced by a new set - isn't restricted to youngsters, of course. Adult birds go through the same process annually, usually once the breeding season's done.
Keep it down
A large clutch of nestlings requires more feeding visits by parents. It also creates lots of noise that may draw unwanted attention from predators - another good reason that many birds opt for multiple broods in succession to keep each clutch size down.