Certainly, any citizen of competent parentage should cheerfully give up his seat for a person in need. Meanwhile, in the real world...
It's a (planned) knock-on effect of ADA or similar laws
ADA stands for "Americans with Disabilities Act" but the EU and most civilized countries have a similar law. It requires accommodations be built-in when facilities are new or remodeled, assures medical privacy, etc. Some laws such as California's Unruh Act also provide consequences for discrimination.
A keystone of ADA etc. is that providers must always "do what is easy".
The very picture postcard definition of "What is easy" is giving the seats nearest the door to the mobility-limited. This is a mandate, so it is required for staff to do this, and the mobility-limited or any ADA lawyer can raise all manner of hell if they don't.
No seat markings are required, so far. Staff must do this regardless. It's mandatory, not a maybe or a suggestion.
But try to explain that to the public
The driver can't really move the bus until the wobbly mobility-limited person is seated. However, she may collide with the manners-limited hoi polloi who frequent transit buses. If she can't get a seat easily, the driver has to intervene. The citizen may retort "But why should I give up my seat?" "Because ADA" will get a blank stare and a digging-in of heels.
So by adding the signs, it preloads the driver's argumemt. "Because you chose to sit in a seat in which those with disabilities get priority, and here's one." Problem solved.
Also: When the grumpy refuser boarded, that there were seats available in the back of the bus. Now, there are not. The refuser could argue "therefore, you should bump one the latecomers in the back so I can have their seat, since I boarded before them". This too gets defused by the sign. "Your eyes were wide open; you knew when you sat down that these seats came with risk of eviction."
What I've said so far applies to all vehicles, even magic vehicles with equal access to every seat (which I don't quite believe).
Indeed, this "priority seating" signage assists with the endless challenge faced by every bus operator: To get people to stop millimg around the doors and move to the back of the bus.
Seats are not equal
On buses I ride, about 30-40% of the seats on the bus have these "priority seating" signs. That's more than you are likely to have mobility impaired people.
In many cases, the seats aren't even slightly equal: it is a kneeling bus, which can deflate its airbag suspension to put its front entrance about 8" off the ground. That is a great aid to the mobility impaired, and it makes the frontmost seats most desirable. This also allows them to board/leave in view of the driver so he can observe or assist.
Furthermore, some seats (4-6) typically "flip up" to create an open space where there are wheelchair tiedowns. Those are in specific locations, and absolutely, get those signs.