The mainspring was the first great mechanical invention. Romans sought to improve upon the mechanism the Greeks used in a water clock. In other words, the goal was to produce a clock that did not need large amounts of water. Leave it to an emperor to require a device intended as nothing more as spectacle that ended up changing the world.
Mainsprings have to be wound, but refinements since the ancient design have made them relatively inexpensive to create. Magic may be simpler, but it is often more expensive.
In addition to timepieces, mainsprings have served as power sources for a variety of things from weapons to automatons to aether flyers and more.
Mainsprings are limited in that they can deliver a lot of power in a very brief time or a very low level of consistent power over time. Despite almost a century of construction, finding a happy medium is elusive.
In addition, the time and resources required for winding larger mainsprings can be daunting. The two mainsprings on an aether flyer used for escaping the earth's atmosphere can require a team of horse and more than a week to rewind each spring.
The next great advance will be the steam engine. The steam engine will prove smaller and easier to maintain. Instead of endless winding, various fuel will be used. Again, this will end up being cheaper than magic, though magic doesn't leave coal dust in the air.
Aether flyers will still have a mainspring as a solar boiler has a more limited range than the mainspring. (And mainsprings will power a ship through an aether vacuum with no problems.) Steam engines can accelerate at a faster rate than mainspring engines.
Steam engines will also allow for reliable mechanical flight. Orinthopters were dangerous at best and quite large. The mainsprings required severly limited their range. With steam engines, however, small flying machines can be made relatively cheaply.
The steam engine will lead to the creation of the electrical battery. Beyond the battery will come the electric engine and the transistor.