Two of my Schumacher's are 6 volt and 12 volt. The only 12 volt battery maintainer I have is one on clearance from Princess - for $15.00 bucks a cheap insurance package. The Schumacher's are usually around $39.00 - not cheap but when they come on sale it can be from $19.99 to $29.99 so that is when I get them - hopefully for the $19.99 price.
The reason I use them is pretty basic - the weather. Up here it can get cold. Not as cold as when I grew up - back then -60F was normal in Northern Ontario. Here is gets to -35C a number of times during the winter with wind chills even colder. It is also a damp cold... brrrrrr. But I love winter so that is cool
One of the things we have always had to deal with was trying to start our cars/trucks etc., in the winter. Usually had battery blankets, block heaters hooked up to help keep the battery warm and the oil fluid. Warm batteries and warm blocks make starting a whole lot less difficult.
Battery maintainers do something similar. It keeps the battery at full charge and whilst doing that there is circulation in the cells which helps keep the battery warmer. Inside a barn out of the wind, this is a big help. Before getting the maintainers I would have to put the battery charger on to help start Ellie. Since I have the maintainers, I no longer worry about difficult starting in the winter. Pull the starter and poof, Ellie is running. Of course this is predicated on all the other start factors being in good shape. I haven't had a no start issue due to a battery in about 3 years now since I have the maintainers.
They are good insurance, don't use much power, cost little and best of all they work. Oh yeah, for those of us with CRS - we don't have to try to remember to go out and start all of our tractors once a week or so.....
Of course as in all things this is JIMHO.