For example, Apollo 8 was the first time a Saturn V (and command module) was sent all the way to the moon, and it was done with a crew. Because there was no lander, there was no backup in case the command module had a problem. If the explosion on Apollo 13 had happened on Apollo 8, the crew would have died in space and never returned.
Remember also that Apollo 8 orbited the moon--it wasn't just a free-return trajectory. The command module had to fire to get into lunar orbit (for the first time ever) and even more importantly, fire to get out (also for the first time ever).
Apollo 8 was originally supposed to have a lunar lander--everyone felt safer with a "lifeboat" just in case. But delays on the lander program meant that they either had to delay Apollo 8 (and miss the end of the decade deadline and maybe the claim to land first) or fly without. The safe course was to delay, but NASA decided to take the risk.
The magic of the Apollo era is that they made it look so easy that we forget how hard it was. The tragedy of Apollo 1 highlights that even simple things, like testing a new capsule on the ground, are incredibly risky.
Apollo 6, the second uncrewed flight of Saturn V was almost a disaster. The booster vibrated badly because of engine instability, and two second stage engines shut down early. But on the very next flight, they decided to send it up with a crew. This would be the equivalent of putting humans on board the next Starship test launch (IFT-4).
Sure, the timeline seems incremental, but only because the dates are omitted. Mercury 1 was in 1961 and the first moon landing was only 8 years later. In contrast, SLS started development in 2011, using existing Shuttle engines and solid rocket motors, and the first landing probably won't happen before 2028.