Solar battery trickle chargers are great! They are really one of the easiest ways to keep your batteries charged.
They can also prevent a multitude of annoying to extremely inconvenient situations like:
A dead car battery when you really need to get somewhere (like work)
Running out of battery power while camping
Running out of battery while far off-shore on a boat
No power when you need to use your tractor or excavator
An ATV or quad bike unable to run which isn’t very fun
Luckily, this can all be prevented. Great if you’re off to have fun but maybe you do want a day off and a dead car battery would be a blessing. Still, dead batteries are never wanted.