- Purposeful Meta Game
Again, for Mtg as an example, units and sets can be released with a purposeful expectation of what a meta-game will look like; and it may be that these units are just there as clutter for you to dig through in order to find the optimal choices. They are still choices and may serve some other metagame purpose (a unit that's good against a set of units that are not useful for the intended metagame, intended to snuff out attempts to curb the metagame into something less desirable.)
See Innistrad. They wanted a metagame of Tokens, token-buffers, and some grave-strategies for longevity; but in case it was too much they included Rest in Peace, Deathrite Shaman, Grafdigger's Cage, Dryad Militant, and other grave-hosers in standard (over time) in order to be sure that the non-grave decks stood a chance. Snapcaster was a high enough power level to keep some level of grave recursion on the map and is one of the obviously best cards ever printed. Note that this ties in with Financial Incentive above.
- Legacy Units and Power Creep
By power-creeping here and there, things are "exciting" for current users. If everything new is average or below average, or at best a lateral upgrade, users won't be incentivized to purchase it or expand their collection. But if it is grossly powerful (or at least looks grossly powerful) people will purchase a lot of product to get their chase rares, to capitalize on the secondary market, or to be sure they're able to compete in the new metagame.
Even in RTS, they don't tend to add bad units; but rather things that you say "holy crap, I can detect stealth units reliably now?", "I now have a mobile unit that can harass their economy?" etc. It has to be "unbalanced" in some aspect in order for it to be interesting; and hopefully you just balance it in such a way that your old units aren't completely obsolete. See Reapers for Nod in Tiberian Sun. Reapers were a strict upgrade to basically everything. A self-healing, anti-air, good against all targets unit that could create a ball-of-death much easier than before. That's exciting.
Stealth tanks, tick tanks, etc were still useful; but the feeling of having a self-sufficient unit was big; and it was clearly unbalanced in the roles it was made for (end-game ball-of-death stuff.) No more did you use Tick Tanks for that.