I really enjoy games with the resource-collecting mechanic. The act of collecting resources gives players small bursts of happiness which adds to the fun of the game. Catan provides this every time one of your numbers comes up. Splendor provides this every turn in the form of gaining new gems or cards.
The second act in our Catan game happened abnormally early, as Robert had an extremely advantageous spawn point. The third act started when Robert began to upgrade all of his villages to cities. The second act in Splendor started the moment people started to purchase cards. The third act began when nobles began to be claimed.
Catan was relatively lacking in theme, which is why I’d guess it was developed with mechanics in mind. While Splendor has a bit more theme to it, its mechanics could easily be re-mapped to a different theme, which is why I’m guessing it was also developed with mechanics in mind.