For general use, any battery charger, or trickle charger, that puts out between about 1 and 3 amps will do, and you don't really need one with float mode monitoring unless you want to be able to leave it connected for some reason.
As to why you should charge your battery instead of driving it around, there are two issues. One is that the alternator can only put out a limited amount of amperage, so the battery is likely to still be low on a charge if you only drive to work or run some errands. The other issue is that alternators aren’t designed to charge completely dead batteries.
Miguel Co / Lifewire