Figures adds content to base. But it does something more important than that. It actually balances base, fixing the issues that often funneled decisions in base.
Base is very much about controlling the first three achievements. The disappointing thing as you get better at base, is, you learn that your early plays are pretty much identical to computer code that was given the goal of "maximize the odds you claim the first 3 achievements."
There's some obvious reasons controlling the first three achievements is important. But the reason that it's so good you don't want to trade them for ANY amount of techup or icon advantage has to do with some other things going on. Pirate Code and Navigation are weak against 1's in score pile. The only answer to 1's in score pile is Vaccination, but it gives away free techup, which is often one of the areas the early achiever sacrifices. 2's are a close runner up. Navigation can't steal 1's, and 2's actually PROTECT you from losing 3's. Pirate Code is really weak against score piles cushioned with 1's. Evolution and Chemistry are hard to use without low value cards in your score pile. Statistics punishes players who don't rely on 1's and 2's. Both players want to score low value card early, and being the one lucky enough to achieve first by turn order or faster scorers has a really good edge.
Figures helps a lot. The achievement action is punished by your opponent drawing a figure. This effect usually isn't enough to deter you from taking an achievement, when given the shining opportunity. That's not how it helps is, it now makes you think about whether achievement rush is worth it. In base, if scores are your 2 to your opponent's five, and you have Agriculture on your board and Math in your hand, best play is to return Math and achieve Prehistory, hands down. Now, if you return the Math, you definitely still want to achieve Prehistory. But it's a little tougher to decide whether you want to return Math at all. The other factors on the board would decide that, it's a more complicated decision in figures.
Later ages have better figures, which solves the issue base had with techup being too weak. It's pretty cool to get some age powerful age 5 cards, but ages 6 and 7 are quite the no man's land. Figures makes it a more certain thing that teching up has some reward.
Including echoes introduces balance issues similar to those of base+echoes; figures in the sand has difficulty helping fix those. Echoes always felt like additional content that I wanted that came with balance issues I had to stomach. Flute is an age 1 four for one, for goodness sakes. Watermill, Almanac, and Clock all play "who's gonna draw me first" hardball long before Industrialization can even be drawn. And I agree with the pro players that say the low age echoes cards are much more pivotal than Industrialization, but Industrialization is a one reason that toggling "add extra achievement" is little help. These cards and several others funnel decisions by obviously being much more powerful than any base card on top, and by being leaps ahead of fellow echoes cards, even. Figures 1-inspire for turn limit guarantees that figures never totally funnels all of the decisions (many inspire effects are humble enough not to need the limit, too).
Just some stuff that was on my mind.
People say "these cards all look so OP", but I always play the skeptic until I see a card behave in a consistently powerful way. My experiences so far have frequently left me unsure which figure to meld, and whether to ignore the figures entirely. You can just score your figure away with Philosophy and use figures as a counterweight help for your base-based strategy if you want to.