What it fixes
Ad blockers, browser privacy restrictions and consent-related data loss. Conversions are sent from a server rather than the visitor's browser, so fewer of them disappear.
What it does not fix
Double counting, add-to-cart set as a primary conversion, or missing conversion values. Those are configuration problems and they survive the move to server-side perfectly intact.
When it is worth the work
Once spend is meaningful enough that a 10–20% data gap changes decisions. Below that, the effort is better spent on getting the basic setup right — which is free.
Start with the double-counting check.