My experience with glow engine models is that the tanks are made to be opened and resealed; the user needs to be able to replace the tubing inside the tank (which deteriorates over time due to methanol and nitromethane exposure) as well as in some cases change the length of the flexible "clunk" pickup tube to optimize for tank position and type of flight regime (trainer, sport, aerobatic, or pattern?).
Additionally, this appears to be a metal cap, so it should be capable of handling enough tightening torque to slightly compress whatever seal is inside the cap. When the tank pressurizes (assuming you're running pressure), the tank pressure will slightly push the cap away from the tank body; there needs to be enough compression on the seal to stay sealed in this state.
Prompted by comments and the edited question: tank pressure, also sometimes broken down to "muffler pressure" or "crankcase pressure" is a method of pressurizing a model fuel tank to ensure high G maneuvers (tight loops, for instance) don't starve the engine for fuel exactly when you need power the most. By pressurizing the tank (typically to only 0.1 bar or so) the tank pressure can force fuel up some distance, even against, say, 20 G. For trainers, it's unneeded; for most sport aircraft, very optional, but for aerobatic or pattern flying it's almost a necessity (last time I flew a lot, pattern models were starting to use actual fuel pumps for this).