In which cases do transactions have multiple "from-account" but a single "to-account"? Taking a sample block on blockexplorer, I see that the generation reward is split between lots of recipients, I figure this is the pool that solved.

But looking at other transactions, there a few with over 100 'From' addresses, and a single 'To' address. The only reason I could think of, is merging many wallets into one. Since I see this in every block I am looking at, I wonder if there's another use-case.

Thanks!
First, if there is more than one 'From' address in a transaction, all of them necessarily have to be controlled by the same entity as each private key is required to sign the transaction.

Therefore, I see a few possibilities:


Somebody is consolidating a lot of small sums from different addresses to a single address owned by himself. This could be for example an Exchange moving deposits to cold storage, or a merchant preparing to pay his wholesale dealer. Alternatively, somebody needs to prove ownership of a discussed amount and moves everything to one address in order to sign a message proofing his wealth.
Somebody wants to pay a large sum, but doesn't have a single address that contains the full amount, so they source the payment from many smaller amounts.
Somebody is withdrawing a larger sum from an Exchange, Gambling Site or Online Wallet Service that hadn't been deposited in bulk.