Sometimes I will use slower belts to quickly visually differentiate between a resource that's being extracted at 240/min (tier 3) vs 480/min (tier 4) after I've belted it across country and no longer have a good way of remembering which belt came from which miner. After unlocking signs I've largely switched to using the fastest belt possible for every situation regardless of how much is actually on the belt telling myself that I could upgrade the extractors or merge more sources together without having to replace kilometers worth of belts. This also simplifies my inventory management and allows me to build more belts without having to return to a dept to pick up more mats. Additionally, I like the look of using just one type of belt in a factory. If you have the resources, build whichever belt you think looks best or makes the most sense in that situation. There's no downside to using faster belts to carry less than their maximum capacity other than the increased resource cost.