Bonnet and Boot gas struts are essentially shock absorbers for the bonnet or boot of your car. They are angled in such a way as to provide an assisting force when opening the bonnet or boot but this force is easily overcome by the force of closing. So they provide both a soft closing action but also take much of the weight of the boot or bonnet.
Over time these parts can wear out and should be replaced, they are generally quite an easy DIY fix.
What problems can be caused by a faulty Bonnet / Boot Gas Strut?