If they were all online, it would be better to just send aid, because the market has fees. Why pay a fee when you can send aid for free?
This is mainly for the benefit of families who's members are not all in the same time zone. lol
Use this as an example...
Player 1 produces iron only, before he logs off he sets a bid on the market to sell 1,000,000 iron at 2.00gc (he wants to help the other members of the family). There is a 2% fee for this transaction which will consume 20,000 iron. The market now has 980,000 iron for sale at 2gc.
Player 2 is a small member who just started out in the family. Nobody else is online to ask for aid, the cost of iron on the regular market is 20gc. This player could go to his family market and buy the iron he needs. The cash stays within the family and is still put to good use.
If Player 1 withdraws his bid on the family market, there is a 10% fee.
It seems like a very effective way to distribute resources within a family instead of sending them as aid to a member and hoping he distributes it in a manner that is best for the family...