i think no ecological study is done on it recently to find exact functional causal relationship explanations on why the subfamily has such prominent spines with energetic costs, but here are some valid hypothesis:
Some spiders, especially Gasteracantha and Micrathena spp. (Araneidae), are covered by a leathery integument and an armature of strong, sharp spines. In some cases, where the spiders are cryptically coloured, these spines may help to break up the outline of the abdomen; but in others, where their owners have brilliant colours and hang, always exposed, in the centre of the web, it is probable that their function is defensive (Peckham, 1889; Robinson, 1980) perhaps by rendering them relatively inedible to birds or too large to fit into a wasp’s nest. Of course, no defence is perfect and some do get eaten. It is also possible that spines may be a cooling device much needed by a spider that hangs on its web in the sun. No doubt the spines have more than one function.
A review of the anti-predator devices of spiders https://archive.org/details/bulletin-british-arachnological-society-10-081-096/page/n3/mode/2up
another subfamily
Species with extremely long spines evolved at least eight times in the [Micrathena] genus and we suggest that this may be related to antipredator defences.
Phylogenetic analysis of Micrathena and Chaetacis spiders (Araneae: Araneidae) reveals multiple origins of extreme sexual size dimorphism and long abdominal spineshttps://academic.oup.com/zoolinnean/article-abstract/166/1/14/2629156