I see plenty of parents with small kids in public transport, those that are using strollers don't even need a ticket. I even remember one co-worker telling that when he moved to Helsinki, he tried using car once to bring his kids to school & coming to work after. He switched to metro after that one attempt and said it made things much easier.
Furniture, materials etc. are not usually things you are transporting daily (or even monthly). Personally when I needed those, I just either paid for home delivery or rented car for few hours.
I won't claim that there are no need for car (e.g. there are some routes that take a longer with public transport compared to your own car & there are ares which don't have many connections available), but it's not as critical as some people might think.
Most people in Helsinki live in apartments so construction etc is not an issue. If you can afford a house in Helsinki, car ownership is not a significant chunk of your budget.
And I have plenty of friends who do have a car for kids etc, but only one car per household. Which is not viable in many places in the world or even elsewhere in southern Finland.
Cars in Finland have not only a value-added tax (VAT) of 24% but also a separate car tax slapped on them. At the extreme cases this can result in over 100% additional tax to be paid on top of the retail price of a car.
One car household is still much more common than two cars per family.
Once you get a family or buy a house, owning a car is basically a must. And the price tag for it in Finland is very high. 1000e a year just for mandatory car tax, mandatory yearly checkup and liability insurance. ~70% of the price of gas is taxes. Cars themselves are so expensive that Finland has one of the oldest car fleet in the developed world (12.2 years).