Why and when would one use such charger?
To my mind, the major needs fall into two major categories:
You have a vehicle that does not get driven enough to keep the battery up to charge against its passive drain. Common examples would include the summer-only car or the utility vehicle that you only use for heavy hauling. In either of those cases, I would tend to hook up the battery charger at least an hour before I really needed to drive the vehicle, just to avoid the potential frustration and delay.
You have ever had a battery fail on you (or worse, your spouse). Traditionally, this is when most of us tend to buy a charger: immediately after a battery failure in a high stress environment. It's important to remember that a sudden dip in temperature (e.g., below freezing) can suddenly illustrate that a battery you thought was fine is actually not able to cope.
I have a relatively cheap charger (item 2 above applies to me). It was a relatively cheap purchase, especially when you consider the stress avoidance factor.
It is not a more expensive battery conditioner that a higher income person than me would leave connected to their summer car's battery through the winter. It is perfectly suitable for bringing my parent's old truck out of their garage after a few months of disuse, however.
EDIT: responding to sharptooth's questions in the comment:
I'm particularly interested in the 2nd scenario. Suppose the car won't
start and I jump-start it and then buy a charger. How do I use the
charger then? Do I charge the battery once and forget or what do I do
You're going to have to make a judgement call. Is this a fairly new battery that died after a month of sitting out in the cold? Or is this an older battery that should have worked but for some reason did not have enough juice to get you going? Or somewhere in between?
In general, the time when I'm most likely to use a charger is when I am in something more like the second situation. I charge the older battery up to full, get the vehicle started and drive straight to the battery vendor of choice.
The times when I am willing to charge up a battery and then run it without concern are focused on the "disused vehicle" situation. In the case of the specific vehicle in question, I bought that battery and I know it's fine. The truck just never gets run unless there's some heavy hauling that needs to be done. In those cases, my procedure is generally:
- Hook it up to the charger
- Eat a sandwich
- Drive away