A contactor is made up of a coil and contacts. effectively you can switch large amounts of currents with 12v,24v,240v,415v supplies. The coil has no current thus if you used a timer to switch it, the coil would draw all of 100mA. The coil acts as a magnet and pulls the contactor in. The contactor acts as a switch, but can switch large amounts of current from 1kw to 20kw or even more. By doing this, all your control circuits are doing is controlling the coil thus putting less load and strain on your control circuits and hence why you would then see much longer life out of these items.
Water pumps are as basic as they come and as you have pulled it apart, it become quite clear how it works. The problem with water pumps in hydroponics is that nutrients build up between the shaft and the surrounding magnet thus locking the shaft in one position. What most people don't understand about water pumps is that they need to be maintained just like anything else in your system would, and cleaning of this shaft and it's magnet are required to keep it running efficiently. By doing this you prevent less floods and problems with your crop.
I hope this helps
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