When done well it adds replay value, when done poorly, it changes the names of some items or skills around.
I asked ChatGPT for fun:
Planescape: Torment – Technically you can pick your class (fighter/thief/mage), but your core stats and growth are story-driven and your combat progression is largely irrelevant; every playthrough converges to the same high-Intelligence, dialogue-heavy “build.”
I think its mistaking metagaming with differentiation, but it's true that you'll miss most of the content if you don't do this.
Geneforge (Series) – You can technically choose between shaping or fighting styles, but progression always gives access to the same suite of powers and creations; no classes, and every player can unlock everything eventually.
Skills that a class is weak in take a lot more points to raise in those skills, but I guess you could. In Geneforge there's three classes: Shaper (the summoner, weak combat/middling magic skill), the Guardian (tank, weak magic, middling shaper skill), and the Agent (high magic). Differentiation comes entirely from character creation, not really what you take along the way.
Deus Ex: Invisible War – All characters share the same augmentation pool; by endgame, you’ll have access to every upgrade.
This is actually a great example of no differentiation. You'll be up to your eyeballs in biomods towards the end so you could swap out augs if you really wanted to, but most of them are trash.
Shadowrun: Dragonfall / Hong Kong (on Normal builds) – While you can select skill lines, most characters eventually blend together because of capped trees and similar endgame gear.
Can someone attest if this is true? This is usually what happens with most modern games that try to have builds or classes, you just path towards the same from a different origin point.