The Wikipedia page on ant mimicry is full of fun facts, but the relevant bits:

Jumping spiders in the genus Myrmarachne are Batesian mimics

Batesian mimics lack strong defences of their own, and make use of their resemblance to a well-defended model, in this case ants, to avoid being attacked by their predators.

Studies on this genus have revealed that the major selection force is the avoidance of ants by predators such as spider wasps and other larger jumping spiders.

But also (not specific to Myrmarachne):

Ant mimics can be myrmecophilous, with the mimics and their ant models living commensally together. In the case of ants, the mimic is an inquiline in the ants' nest. Such mimics may in addition be Batesian or aggressive (predator) mimics. To overcome ants' powerful defences, mimics may imitate ants chemically with ant-like pheromones, visually, or by imitating an ant's surface microstructure to defeat the ants' tactile inspections.