For web stuff in particular, using a comparatively low-bandwidth and/or high-latency connection can also be useful, since not everyone has 10+ Mbit connections with a 20ms ping to a Bay Area datacenter. On Linux, you can use 'tc' to simulate that.
I had a 384 Kbit, ~100ms-latency crappy SDSL connection until recently (was renting in the Santa Cruz mountains in a location that had poor connection options), and it was pretty amazing how two sites that looked very similar from a fast connection, would load much differently on the slow connection, often for reasons not really inherently connected to the site's needs (it's one thing if it's loading slowly due to streaming video, versus due to having a gigantic background-wallpaper image, or unnecessarily serialized roundtrips).
I had a 384 Kbit, ~100ms-latency crappy SDSL connection until recently (was renting in the Santa Cruz mountains in a location that had poor connection options), and it was pretty amazing how two sites that looked very similar from a fast connection, would load much differently on the slow connection, often for reasons not really inherently connected to the site's needs (it's one thing if it's loading slowly due to streaming video, versus due to having a gigantic background-wallpaper image, or unnecessarily serialized roundtrips).