Why use fastdl?

You use fastdl because it's faster than slowdl. How come you didn't know this?

Because relying on your own bandwidth is better than using workshop. Workshop frequently goes down and steam loses connection.

because nobody likes to be stuck in the loading screen for 20 minutes

Almost no problems and headaches compared to the workshop

fastdl is more reliable and it can add files to your player's game if it's not on workshop. Pretty easy to setup too.

FastDL is fast. In downloading files and loading cached files. The problem is only in updating a file that has already been downloaded and a limit of ~ 4k files.

If you want to create a successful server, then yes.

If it's for friends, no.

FastDL is super important for server creators since it's the first thing players are going to experience the first time they join.

Let's compare:
If you don't have FastDL and have hundreds of add-ons and such, it can take HOURS for someone to join. If you have FastDL on the other hand, it can take maybe 3-20minutes. (Depending on the size of add-ons and content packages you have compiled)

FastDL will accelerate the speed of downloads so that new members can join your server much faster, and correctly! They won't be able to download content without it, so if you don't want anyone complaining...use it!

Source: I used to be a hosting manager that sold Garry's Mod servers!

It loads and keep files on client`s computer so they dont need to download files again

If you're running a busy server it also lets you potentially offload traffic to another node, since the files don't have to be hosted locally to the main executable.

FastDL means that users will be able to download server content much faster, and beats other methods of getting materials/resources. If you own a Gmod server, FastDL is pretty much a must nowadays, since it prevents the tedious task of constantly referring players to a workshop link.