Software that circumvents rate-limiting. Axel (still available in Archlinux repos at least) can be told to use some number of connections, it'll figure out the size of what you want to download, divide it into the number of connections you want, and start the connections in parallel, each getting a different chunk. Works when the rate limiting is per connection.
Other rate-limiting strategies like slowing down a connection over time would need other "acceleration" strategies like restarting the connection when it slows down, continuing from where the previous left off.
Another possible option that isn't about circumventing rate-limiting but just speeding up downloads can be to download different chunks from different mirrors, to spread the uplink burden. Axel can be provided multiple URLs for this.
I used to use a download manager because it allowed me to pause a download, turn off my PC, and continue the next day, which made sense in the days of 56k.
In the past (and even today) some servers didn’t enable tcp window scaling, which limited you to a couple of megabit in say a 100ms distance download, not because of any conscious blocking, but because you couldn’t have more than 64kbytes in flight at any one time.
This was mostly fixed a mover 20 years ago when window scaling became default, but companies like signiant actually had demos where they deliberately turned off window scaling to show how their proprietary file transfer formats were so much faster.
In addition to what other people said, they allowed you to resume downloads.
Even if you weren't on dial-up, sometimes downloads would just... pause. Like you'd get to 23% and just wouldn't get any more data from the server. With these programs you could make progress by restarting the download, and it would pick up largely where it left off.
Man. Nostalgia.