My scan gauge earned it's keep by giving me peace of mind (knowing all was well) ...and giving me a hint on the severity of any issues that did arise ... all while helping me to not lug my engine. Too much complicated information is sometimes far wise than none at all....
There are three general classes of "scanners"
One is the ScanGauge (DashDAQ, UltraGauge, etc) ... whose primary feature/use is as "real time as you're driving" readouts of system status: speeds, MAP, fuel consumption, etc.
They have rudimentary P-code readout, and can usually perform a simple bulk Clear of P-codes.
Button placement and what-you're-shown are designed for on-the-road.
Next comes the better "scanners", such as the AP200, the Autel M802 and 808, etc.
These can provide moderately deep-dives into the P-codes and sub-system codes (with trailing qualifier digits). They may only have the simple Clear like the ScanGauge, but they're gradually getting other general clears (airbag (SRS) and DEF).
The AP200 is cheap at $70 (or less), the devices-with-buttons cost $200 and up.
Finally comes the range of "full diagnostic" tools ... ranging from the DAD to Dennis-level SnapOn and MB's Xentry systems .. these are wired (and smart enough) to really reach into the sub systems and perform individual component check-out and control (such as the DAD's ability to step the turbo actuator to any chosen value from 5% to 95%)
The DAD was a price anomaly, since it was used equipment .. its commercial equivalent (the CarSoft hardware/software) was $1500 ... Dodge/Freightliner dealers used the $6000 DRB-III
--dick (has ScanGauge, DAD and AP200 ... each has its place)