Some people are not sure what type of bike to buy. If you buy a 29 inch mountain bike, you can run mountain bike tires, gravel tires, or road tires. Then decide which you prefer.
There may be some 29 inch mountain bikes that are different, but on my bike, the rims are narrow enough to run road tires.
Some people might want to use one type of tire for summer, and another for winter. You can just swap the tires.
If you go the other way, and buy a road bike, the space between the frame is too narrow, so you can't run wide tires.
You mean knobby tires, semi-slick tires, or slick tires?
When you say road, it begins to suggest width of the tire.
'I had tried buying 1.25" Continental Contact Speed for my 27.5" street racer convert. Tires that narrow provide bad versatility for bumps and look gaudy between the wide stays. That's when I opted for the 2.00" version. Which are like balloon tires. I rode them overclocked at 70psi and they were fantastic.
On my mountain bike I have mountain bike tires. If I wanted to ride long distances on good roads, I could switch the tires to narrower road bike tires. These would give less rolling resistance, and enable me to go faster and further.
There are some people with little experience with bikes, who don't really know what type of bike they want. They could get a bike like this, and try different tires to see what they like.
In reality, like you, I am running my rear mountain bike tire at higher than normal pressure, to reduce rolling resistance while on good roads, while still being able to go off road.
The tires that came with the bike are rated at a maximum pressure of 40psi. I have left one of those on the front. I changed the back tire so I can run it at a higher pressure, and will use the original back tire on the front, when the front tire is worn out. When both 40psi tires are worn out, I will probably run high pressure both front and back.
You realize that off-road is better with lower tire pressure?
If you're overclocking them, you're suffering off-road.
I usually ride my winter bike at low 40psi or a little less.
My situation is the same.
I run 26 x 1.5 Specialized Nimbus 2 Sport tires at 80psi. They handle off-road okay so long as it's dry.
Shifting way down adjusts what you need in simple passes.