why having those meters are helpful, in my opinion. They're only good if you can USE/READ them properly.
Vacuum level. this helps in diagnostics. A leaking intake manifold and a lot of other breathing problems can be noted from this. An experienced mechanic can eliminate a lot of possible problems by just looking at how the vacuum meter behaves. If you as an owner notes how your vac meter behaves, you know there's something wrong when it's not reading like it usually does. This is usually for NA only meter, though some boost meter also have the vaccum portion to read off-boost manifold pressure.
Oil pressure level, also helps in diagnostics. If your oil pressure is dropping, then there is something wrong with your oiling system. Leakage, cavitation, etc, sometimes, maybe just a worn oil pump, or if oil pressure drops during a high speed cornering manouver down in sepang, it just shows that your car can rail through corners at speeds fast enough that you need a baffled oil sump. Some of the tactics people use to avoid this is by OVERfilling the oil sump so that cornering Gs that cause the oil to slosh about is minimized, though at the expense of having the dipstick popping out sometimes, and losing a bit of HP, which losing a little HP is not a problem for high boost turbo cars. VTEC cars suffering from 'my vtec cannot open' can also benefit, by noting the pressure during vtec engagement, which is 40psi minimum, so instead of running about figuring what the problem is: (solenoid, pressure switch, etc.?) though with chipped ecus, this can be bypassed, to the detriment of the engine, IMO. For turbo cars, low oil press means a soon to fail turbo, either from coking, leaks, etc.
Oil temp, prolly more important to turbo than NA, because turbo using the oil to lubricate, and to a certain extent to cool the turbine bearings/bushings, so unlike NA, the curve which temp rises for oil and water is not the same. For NA, apart from the initial warmup, oil and water almost same temp, just a few degrees apart. Hot oil means it's starting to thin out, and that also means less lubrication. It can also give you a hint if your turbo is overheating or not.
water temp, a cheap way to monitor engine temp, and the sensor practically lasts forever too. Its so you know when the engine is warmed up enough for hard driving.
Fuel pressure, a way to monitor fuelling system. If your fuel pressure is dropping, means something is wrong with the fuel pump or fuel filtering system is clogged. time to change/service. You can base this on mileage rather than using the meter though, so this is more of something for the meter 'enthusiasts'. Though it can also be considered a tuning tool, in the old days, fuel pressure = fuel adjustments, before the days of EFI, when carburetors are norm. EFI also relies on fuel press to a certain extent, but the importance is a lot less than carbs, so for EFI, it's usually set and forget.
Air fuel ratio, a way to monitor a lot of things, but you need to know what the numbers and graph it can output means. It can give clues and hints on misfiring, engine knocks, fuelling or even how fuel efficient your car is. there are two types of meters, one is the A/F ratio that connects to the stock sensor, which tells you nothing, except if your closed loop system works or not. On a car that is emissions friendly, it should register stoich/a little lean at all times at part throttle, and rich at full throttle. A wideband read more sensitively, and can give hints on a lot of other possible issues like I stated earlier.
Exhaust temp is a way to measure A/F ratio. It is a cost saving method of not having to connect a wideband 02 sensor (which is EXPENSIVE, and spoils a lot more often than the exhaust temp sensor) On a heavily boosted turbo car, wideband 02 sensors spoil quite often compared to exhaust temp, which is why it's a good substitute.
By looking at the exhaust temp and correlating to an A/F meter (which later can be removed), a car owner can notice when the exhaust temp is getting colder (which usually indicates the car is running lean, and stop driving aggresively before causing critical damage (on high performance engines, because stock engines don't see the stresses like modified engines do). Due to the relative cheapness, some budget racing teams mount four or more exhaust temp sensors so they can do individual cyclinder tuning.
For a four cyclinder, 02 price is probably not a concern since it's abt. USD100x4, but imagine those owners with v8-v12s, OUCH!
esp. those with domestics v8-v12. not all american domestic owners bathe in money.
The more extreme the modication to the engine, the more likely things can go HORRIBLY wrong when one component fails, so most racing teams/modified car owners who are technically inclined use these meters to indicate first signs of trouble, and replace the part before critical failure.
Of course, Malaysian people like it because it looks BLING!, which is nothing wrong really, since these people are the ones that buy the most meters, which keeps manufacturers happy and fund their R&D.