It's a limiter. You can use Uranium to build a Nuclear Plant, an Atomic Bomb, a Nuclear Missile, or a Giant Death Robot. If you use a Uranium for the Nuclear Plant, that's one less Uranium to use for units.
Similarly, Aluminum is used for Hydro Plants, Spaceship Factories, Helicopter Gunships, Jet Fighters, Missile Cruisers, Mobile SAMs, Modern Armor, Nuclear Submarines, Rocket Artillery, and Stealth Bombers. If you want to put in a Hydro Plant (or a Spaceship Factory), that's one less Aluminum available to go towards your military.
It's all part of the apparent design of "millions on defense, thousands on offense." You can have as many (for instance) Mechanized Infantry as you can support (subject only to the "# of cities + # of citizens + game-difficulty-modifier" limit), but if you want anything more, you're going to have to make hard choices.
Solar Plants only require the city border (or be on?) a desert. No (known... so far...) requirements for strategic resources. However, if your city has a desert tile within the "first ring" of city radius, it probably has a LOT of desert tiles in its radius. So you're getting compensated for already-bad terrain.