Both activities get products into your store.
One should be automated, the other one needs a bit more intervention from you.
Purchasing is a formula built using a few variables- stock status, historical purchase, sale’s growth (or decline) etc. You should have a system that tells you this automatically. Your time is better spent elsewhere.
Sourcing is a bit more intuitive. A bit of trials and errors, hits and misses. Having a budget will also help reduce the risk. You get better the more you do it.