Simply put, you should not run your laptop connected directly to the wall socket. A laptop power supply AC adapter brick is typically a simple AC to DC step down converter. Inside you will find a transformer and a bridge diode assembly which drops the 120 VAC to something like 20 VDC and a couple of capacitors to try to reduce ripple. The problems are as follows:
Say you have a brownout or low voltage condition which extends for awhile. The undervoltage causes a drop in output voltage which can cause your laptop to have issues dependent on whether the battery is installed. If installed, it should run from battery and eventually drain it down. If it doesn't, the computer may continue to run, but overheat due to the fans running too slow, and other devices starving for voltage. You may have errors, data corruption...
Say you have an overvoltage - it will cause the dumb bunny AC brick to pass too high a voltage - kind of the same issues - burned out circuitry...
Lastly even a full UPS is cheaper than some of those bricks, it is cheap insurance to use one. At least, use a surge protector.