We do not use or wear them. A person who undertakes the 8 precepts or more is not supposed to wear a mala, as the 7th precept includes 'mala'.
Nacca-gita-vadita-visukkadassana mala-gandha-vilepana-dharana-mandana-vibhusanathana veramani sikkhapadam samadiyami
I undertake the training rule to refrain from dancing, singing, listening to music, going to see entertainments, wearing garlands, using perfumes, and beautifying the body with cosmetics.
mala is commonly translated as garlands but the pali text clearly says mala.
They can be useful throughout at a certain time in your course of vipassana meditation to count certain arisings, but are by far not necessary for this purpose.