Turmeric is poorly absorbed by the body. This is why the combination with black pepper is interesting, as they work hand-in-hand, because the spicy component of black pepper - the piperine - improves absorption.
Scientific evidence
Piperine is the spicy component of black pepper. It provides 20 times better absorption of turmeric into your body. Not insignificantly, black pepper contains only 5 to 9% piperine. So this means that more than 90% of the black pepper you use in cooking is not piperine, moreover, piperine is sensitive to heat. So supplementing it is an absolute must.