Not sure of your AO, but if you are in earthquake country, storing your gear in a barrel allows it to be easily found if your house is trashed (paint it blaze orange) and prevents your critical gear from getting crushed if a wall falls on it.
Before you use it as a rainbarrel, you want to clean it out as much as possible. When you think it is completely clean, put water in it, then test the pH of the water...compare to your tap/well water....if significantly different (usually if soapy it will be higher pH)...don't use it for that purpose.
If metal, should be easy to clean, if plastic, you may not be able to completely clean it out sufficiently for water usage.
Usage - possibilities are smoker (preparing food for long term storage!), water storage, gear storage container, water filtration cannister, solar still, solar oven/water heater, form for concrete pillars, the possibilities are endless...
Don't forget the ever-popular steel drum...start your own band!