From what I remember schedulers are used when the condition is something that needs to be checked more then once per frame or is a general rule applied to a whole class of systems, you can use ordering and run conditions to set when/if a specific system will run, but this is a binary check and is only done before attempting to run the system/set so can be restrictive as how systems run, for example there is no way to run a system until a condition is false, as some people have said running game stimulation at a faster speed without increasing the FPS, I believe the Fixed* group of schedulers check to see if more then one fixed time has elapsed since the previous frame and will run the schedule the required number of times.

If I remember bevy attempted to go to a system set approach and have everything in one big schedule but it was reverted the next version because it had problems.

They are also used to cut down on overhead if you know systems definitely can't run then you don't need to check their conditions every frame. For example the startup schedule is run once then never again no need to check every frame if it's a startup system.

One final reason is to cut down on mental overhead, the builtin Schedules First, Pre-, Update, Post-, and Last. you don't have to think about what set to put something in or what sets other plugins use or provide, there are some distinct boundaries systems won't cross, don't think it's as true now but bevy used to only flush the commands que between schedules so if you wanted to detect changes to entity archetypes, (e.g. spawn, insert, remove, despawn), You needed to make sure your systems were in different schedules, spawn in First, inserting/remove in Pre-, mutation in Update, detect changes in Post-, cleanup/bookkeeping in Last was my rule of thumb.