so in one of my previous posts i had mentioned that i would make an entire thread devoted to plant 21 (2 coal/oil -> 4 cities) and why my friend & i rank it in the top 10 overall on USA/Germany style maps (i.e. a "normal" resource structure with no extra gimmicks that would affect things). it's not just us, either; the best online players will pay around $40 for this plant (and the also-controversial 20 plant, as well) a decent amount of the time! this tends to be the most contested of all of our rankings, as people often say "it's not an endgame plant outside of 5-6 player and there are more efficient 4-caps, so why bother?"
the answer can't easily be summed up in a single sentence. the 21's strengths are the most multifaceted of any plant - i like to call it the swiss army knife of power grid plants! so let's break this down in detail...
the first key factor here is that the 21 is the lowest-numbered 4-cap plant and also has a lower number than all but one 5-cap plant. when you compare this to the other 4-caps that look better on paper (28 & 29) it's a wide gap with some very important plants in between. having turn-order advantage over the 25 & 26 (the absolute 2 best plants in the game imo, as discussed before) is a huge deal, and there's also the 24 (good in 5-6 player) and even the 27 somewhat (decent in 6-player or if it comes onto the market very early).
probably more important than turn order, however, is how much earlier the 21 becomes available on average than the other 4-caps of interest or the vast majority of "endgame" plants. you can often get this plant during step 1, and i think that more than makes up for having to replace it at the end. obviously, the earlier you get the plant the more money you'll make from it...but there's also the fact that you are preparing yourself well for the midgame plant market stall by grabbing the 21 early on. this is one area where the "capacity is king" mentality promoted by many players on BGG and this blog can end up biting you in the ass - if you go in hard for the 25 or 26 and miss out on it, then ignore this plant, you could very well end up stuck with nothing while staring at a row of plants in the teens for the next 3 turns! or alternately, you could hugely overpay for one of those 5-caps because OMG I GOTTA GET AN ENDGAME PLANT NOW, when you would've been better off stopping and just taking the 21 for much less (speaking from plenty of experience on this one!). in these types of scenarios, the extra money you gain from the 21 will definitely make it worth having to buy another plant at the end.
the other factor that makes the 21 so strong is its combination of being a hybrid plant and using 2 resources instead of 1 or 3. this is where the "swiss army knife" element comes in - you can "play defense" by buying the cheaper resource if there's a big gap between coal & oil prices, or use its storage capacity to buy a bunch of the scarcer resource if this will attack a threatening player. though the 29 looks more appealing than the 21 with its efficiency, it really isn't much cheaper to run if you go the defensive route, and it's much weaker for offense since you can only store 2 pieces on it instead of 4. given that it's better to buy the more expensive resource for your hybrid the majority of the time, i give the edge to the 21 here as well. these properties also make it a useful storage plant for the last turn, and having extra resources for the end gives you a lot of extra flexibility in choosing your final plant!
so to try and condense all of this into a single paragraph: the 21 has excellent turn priority given its capacity, and it tends to show up early enough that it will more than pay for your last plant while putting you in a very good to great position when the plant market stalls in the midgame. the latter part also applies to the 28 & 29 when those become available that early in the game, but that happens significantly less often due to their much higher numbers. the 21's medium efficiency and hybrid status allow you to run it at least fairly cheaply or strongly attack a scarce resource as needed, as well as stockpile resources for the last turn, and that level of versatility is unparalleled in this game. put it all together and you have a plant which can be worth double its starting price even in in 3-4 player games, if it's early enough and you know the market is going dry.