Seatposts, at least when they’re static, have a set-and-forget quality. Every bike absolutely needs one. You get a new one with a new frameset, and you keep it for a long time, so finding the best seatpost for your needs pays off. Seatposts don’t need to do much, static ones typically don’t wear out, and dropper posts are usually built for abuse. Upgrading is the main reason to replace a seatpost, and there are pretty much only three reasons to upgrade. First, to lose weight. Second, to add compliance or reduce vibrations from riding. The last reason is to either switch to a dropper post or from a dropper post.
The concept of dropper posts has been around since the early days of MTB when the Hite Rite provided a rudimentary drop and raise. The modern dropper first gained adherents from big-hit MTB disciplines and has moved toward cross-country, even finding a place in gravel and occasionally on the road. Modern seatposts are typically hydraulic-activated, with the hose internally routed, and offer much smoother, more reliable movement. Still, there are cable-actuated droppers, externally-routed droppers, and even wireless droppers—RockShox Reverb AXS. If weight is a concern, the KS Lev Ci or Fox Transfer SL dropper posts are amongst the lightest.
Carbon seatposts are typically associated with dropping weight from the bike or adding compliance. Sometimes both. Lots of super-stiff bikes are equipped with posts that have some fore/aft given to them. And if they don’t, it can be a good idea to add some compliance; your back might thank you. ENVE and Zipp both make some super-light seatposts, but they’re of the round variety and work with a few standard diameters. Smaller diameter posts are more compliant, so a 25.4mm seatpost has more give than a 27.2. Canyon has a funky compliance post, the S15 VCLS, that is light and works with round seat tube openings. Many bikes come with proprietary non-round posts that are supposed to aid in aerodynamics, offer compliance, or both. If your bike comes with an aero seatpost--airfoil, oval, or D-shaped--you’re most likely limited to posts from your bike’s brand, possibly that same model.