A size chart is a table of garment or body measurements. It is accurate, but it puts the work on the shopper: they have to know their own measurements and understand how a garment is meant to fit.
What a recommendation adds
A size recommendation takes a few simple inputs, such as height, weight and fit preference, and returns one size for the product the shopper is viewing. It removes the translation step between a chart and a decision.
You still need good charts
A recommendation does not replace a size chart. Some shoppers want to check the numbers themselves, and clear garment measurements are useful data for any recommendation method. Keep both on the page.