Purpose of spring for heatbed?

It has been a long time since I follow the news of reprap since my repstrap extruder was destroyed. With zero power tool and all workshop refuse to drill a 0.5mm hole I just gave up. Until I found that there are some reassembled JHead+Wade's on a local bidding site, that reignites my interests. And that was when usage of heatbed was just being investigated.

What's the purpose of using spring for fixing the heatbed - prevent damage when head crash?
I wonder won't it affects the height as the build goes on and becomes heavier?

The springs are pre-compressed by the bed bolts, the print bed won't drop until it weighs enough to match the force needed to compress the springs that far. Which should never happen if you make sure to compress the springs more than a few mm.

The reason is to make it easier to adjust how 'level' the platform is vs the print head. Without it, it would be more of an issue than it is now.

There is only two reasons and it is to prevent your bed from being seriously damaged if your hotend accidently crashes into it and to allow easy leveling of your surface. I've had my hotend crash into my glass print bed plenty of times and the springs have saved me every time. Yea, there is the issue of weight causing the springs to compress but that will not happen if you screw them down far enough to counter the weight of the printed part.

The reason for the springs is to provide an opposing force to the leveling screws. That is all.

They are not intended to save you from head crash damage, although they may buy you a little extra time to kill the power.

Idealy, there would be no need for bed leveling, but due to the inherent varaibility of the printed parts in the RepRap design, some adjustment is usually required.

FYI, I used 4 short pieces, about 3/8" long, of silicone tubing instead.

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Thingiverse ID Alan1279

My first printer has springs to the heated bed, my MendelMax I mounted it rigidly, and it's a lot less hassle.

There is also the issue of parts that warp but are large enough that the layers cool down to the point of being solid. It's better to fix that particular problem, but at least with the springs it's a lot less serious when you get hit with this. The nozzle pushes the part down as it moves across and the print is able to continue.