What was the purpose of the plaque belt?

Was it just a fashion item of the times, or did it have some additional significance (e.g., worn only by knights)?

Did serjens wear plaque belts, too?

Fashion/decoration. They saw continuous use from the Bronze Age right through to today.

I too think that they were fashionable. I can tell you from modern experience that they are currently a prestige item because they are prohibitively expensive to produce. That doesn't keep me from looking for a new source though... :)

They were sometimes to often used to suspend sword and dagger.

Are you thinking of the belts worn around the hip by late medevial knights?
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In this case, the large fittings or hinged nature is probably to make a very wide,stiff belt that does not twist or slide.
If you compare it to other belts of the period, they are usually very narrow, and apparently thinner than modern belt leather from the way they hang.
Earlier swordbelts are typically wide and made of lamb or deer hide, which is rather soft. These belts hang loosely from the waist to the hip.

If you look at the placement of the belt above, you will also notice that it does not sit around his waist. And the waistline is a lot higher than modern mens fashion. In fact, his breastplate ends at the ribs.
The belt is secured around the hip itself, not over it.
The advantage of this is that the hip is the part of the body that moves the least when running or figthing. As you can see he has his sword suspended from a single point on his belt. If the belt was carried on the waist this would be quite unpractical as upper body movement will jolt it around. But at the hip it is a lot more stable and practical.

From there, it naturally goes on to become a status item, as it is carried by soldiers AND is blinged out.