Yes, that it was especially rural environments and not having much options otherwise to live around while building.
Sauna that was built then wasn't just one hot room, but it also had at minimum small changing room dressing/undressing, relaxing between turns in steam room. Also if it was first building made then adding also lounge which served as living space with beds and cooking stove while building house was common. With sauna you had place to stay warm first winter, able to get warm water, wash clothes, yourselves and even a give birth old times. Building sauna first made lot of sense.
These days sauna for home builders is more about getting sauna somewhere in that floorplan where works well for the intended users of that house.
Due to lack of running water in those times (and still in many cottages) cooking is done above a fire, water is brought from the lake. A kitchen won't serve you well if you're just trying to get through a long winter of -30c.
While it's true something like 90% of the accomodation have a sauna it's not like everything is planned around it. It's more like that it's the ONLY well soundproofed space, with nice atmosphere, that makes life enjoyable when your neighbors suck.
I have no idea if that claim is true, but what I did love about visiting Finland was the even the small apartment I rented had a sauna in it! It seems like it's a non-negotiable for even the smallest accommodations.
The cheaper apartments tend to not have private saunas built into the bathroom, but most apartment complexes at least have a shared sauna on the top floor. Residents can book a block of personal time in advance.
"Tar, acclaimed to have been formed from the sweat of Väinämöinen, a central character from the Finnish national epic Kalevala, was an important medicament to the former-day Finns. Tar actually did bear antiseptic features, which worked as a cure for infections. Lately tar has been recognised to include parts that can cause cancer, and the European Union has urged that its use should be avoided." [1]
I personally dont know how tar was used for health, but it was big export item of Finland during medieval times.
Vishnevski’s Liniment, which contains birch tar, was a common treatment for wound infections and burns in the Soviet bloc. However, this was something that individuals used because there was nothing else at hand.
Now, there are things like Fucidin, Polysporin and silver ointment for infected wounds and burns, respectively, that are safer and more effective.
Some people still swear by it, because “tradition” and probably some element of malignant patriotism too.
It's mildly anti-fungal as well, which makes it effective in dandruff shampoo since a lot of dandruff is caused by fungal overgrowth, aka seborrheic dermatitis.
Another weird/fun one is using bleach as an anti-inflammatory (topical only, of course...), although these days you can find derivative products that offer the same benefits but are much less harsh.
I take a mild bleach bath sometimes and it’s quite invigorating. Seems to kill off a lot of skin surface bacteria which can sometimes be beneficial (there’s good and bad bacteria on your skin).
Not to be done too often but every once in a while I find it helpful. Not all that different from a strongly chlorinated pool.
Another cool one, especially if you don’t have a sauna, is doing a mustard bath. You will sweat like a stuck pig
Besides water proofing wooden boats and long time ago ships pine and fir tar it's been used protecting wooden roof tiles when they were a thing and still are used old wooden churches keeping and restoring.
It's used small amounts in additive in soap or shampoo mostly as a scent, mouth pastille and lozenge a for taste, animal health care kind antibacterial and bug resistant etc. long time ago.
Quite lot of applications especially old times long time ago before more scientifically developed medicines were commonly available. These days less there but it's used as a scent or for flavour.
Go to an ER or UC and have them dress a wound for you. They will use a healthy dose of petroleum jelly and generally tell you to stay away from antibiotic ointments.