Xysticus cristatus female on her crab-like stanceUnlike flower crab spiders, which feed mainly on pollinators - bees, hoverflies and butterflies - ground crab spiders feed on a wide range of prey, from small prey such as ants aphids and springtails to the larger bees and butterflies, and other spider species, even earthworms! (see Figure below from Nyffeler & Breene). Birkhoffer and coleagues found that Xysticus cristatus - together with wolf spiders - eat larger proportions of aphids when compared to web building spiders. Their common aphid prey might make them a suitable biological control, retarding the population growth of aphids during spring so the authors suggested to encourage habitats favourable to these spiders near crops.
Nyffeler, M., & Breene, R. (1990). Spiders associated with selected European hay meadows, and the effects of habitat disturbance, with the predation ecology of the crab spiders, Xysticus spp. (Araneae, Thomisidae) Journal of Applied Entomology, 110 (1-5), 149-159 DOI: 10.1111/j.1439-0418.1990.tb00108.x
Birkhofer K, Gavish-Regev E, Endlweber K, Lubin YD, von Berg K, Wise DH, & Scheu S (2008). Cursorial spiders retard initial aphid population growth at low densities in winter wheat. Bulletin of entomological research, 98 (3), 249-55 PMID: 18439342