Short and quick.
The main difference between chargers like those Samlex and cheap chargers is that the Samlex type are a true multiple stage charger. When connected to a depleted battery the electronic controls will place the charger in "bulk" charge. That is a charge where the amperage is held constant, or near so. The voltage slowly rises as the charge builds. When the voltage reaches the "absorb" charge set point the charger changes to a constant voltage charge. (Absorb voltage for a typical lead acid FLA battery = about 14.4. This can vary from make to make. Best to check) Absorb charge is usually a timed charge; 2 hours, 4 hours, etc., at a lower amperage rate. What is needed also can vary between makes and types. The final stage is "float", which is supposed to be just enough to maintain the charge without causing excessive water loss.
Advantages of a good multi stage charger include; faster charge than with a cheapy. Saves gas when using a generator. Batteries have a longer life.
A good one includes a remote temperature sensor so if the battery gets hot the charger will slow down the amps.
A cheaper, non multi stage charger can get the battery recharged but will take longer. Reason for that is they do not have a constant current bulk charge. The amps into the battery decline as the charge builds. That is because of increasing battery resistance. If you are plugged into the power grid long enough the battery should reach full charge.
As with many things there are reasons to have better chargers with the multi stages. There is a 4th stage, equalize, that is used from time to time with FLA batteries. Never with an AGM unless the manufacturer lists some special cases and you follow their directions explicitly.
Short, quick and not a complete course....