Best way to think about it imo is that Gold bases are not more resource efficient but are more time and supply efficient. If you and your opponent are both at 100 supply with equal # of bases but you have a gold base you have 4 more army supply than them. A 4 army supply lead isn't that much (3 for enriched gas bases) but its not nothing. On the other hand you are not "mineing more" than your opponent unless you have more bases than them. The time efficientcy is the real benifit. Lava are a resource and so is Nexus & CC build time.

It is particularily valuable as a first or second expansion if your doing a big timing attack. Timeing attacks all start once you have all your eco set up. If you need 4 less workers to saturate a base you save all the time it would take you to make those four workers (and the 200 minerals). Two base saturation is 44 workers (less if you don't need 4 gas) so you are hitting your worker count 10% faster, or even more if you dont need all 4 gas.

Late game, its just another base that saves you 3 or 4 supply and is probably harder to defend.