Downloader is damn impressive implementation of client - server based programming.
It has again 2 parts client and server. Usually in case of trojans, server is installed on Victim's machine and client is installed on Hacker's machine. But in case of downloaders. client is installed on victim's machine and server is installed on hacker's machine. The purpose of the DOWNLOADER is to have control on remote machine for longer time.
Downloader whenever gets installed on a machine, it starts transmitting the information regarding the victim to Hacker. It opens the port on victim's machine, so that hacker can communicate through those ports.
When a connection is made between server and client, now hacker can send various viruses or wroms or whatever he wants to send to your machine easily through downloaders. It helps Hackers to always keep their harmful programs undetectable from victim's anti-virus programs because hacker will always keep update the code of virus with the time and will make sure that victim's machine has the latest virus.
Downloader can result that your machine is attacked by various kinds of viruses or worms at the same time. Downloader itself is just a carrier program which helps for other trojans to installed on your machine. Its like making a road to your machine from hacker's machine, through which different-2 viruses are served to you.