You use LSMW to load master and transactional data: cost centers, G/L accounts, customers and vendors (business partners), materials, open items, and more. It is the classic tool for the one-off data loads every SAP go-live and rollout needs.

Is LSMW obsolete?

Short answer: no — but read the room. In S/4HANA, SAP steers you toward the newer Migration Cockpit (LTMC / the "Migrate Your Data" Fiori app). LSMW still runs and is still used every day, but on a greenfield S/4HANA project you will often be expected to use LTMC. We cover that move at the end.