harmful effects: exactly none
That's plenty enough "exercise" and only the oldest battery would have trouble keeping a charge between runs. Far worse would be to drive it more frequently, but on little half-mile jogs that don't give it a chance to become fully-warmed-up (which reduces moisture and contaminants in oil, other fluids, boils water out of muffler, etc).
Just follow manufacturer's recommendations for maintenance: while many recommendations are based on miles driven, other things are based upon time - for example, engine coolant will lose it's anti-corrosive capabilities over time, whether car is driven or not, and needs to be replaced per recommendation.
FYI: Have owned 50+ cars over the years and driven some of them 150,000+ miles (and disposed of some lemons at 2500 miles !) . . . . but am retired now and relocated to a condo where nearly everything is within walking distance. I am driving my car only about 500 miles a year . . . it might sit for a month between uses, but when I do use it - say to go to the supermarket a block away - I go "the long way around" to fully warm it up. It's a 10 year old car now, like new, hardly 6000 miles on it - - -- and it goes to the shop once a year for a complete check-up: using synthetic oil now that we change every other year . . . . battery replaced when it was 7 years old . . . . anti-freeze every 2 years as recommended.