Meaning I can know in advance whether the incoming transaction (API or form) is up to par with my requirements, at least based on what I've read so far. Ok, I'll start investigating the technicalities of this. Thanks to everyone commenting here 
Why would someone use this service instead of just broadcasting that TX to the network ?Because broadcasting the message to the network [as I understand it] requires the user to be connected to at least one peer in the network, meaning a different kind of protocol to handle.
Having a service that will broadcast your transactions to the network for a small fee (if the transactions are not small ones) requires you to compile and sign a transaction, and send it to the broadcasting service via HTTP, very straight forward for client-side clients and thin clients.