Every PC needs some kind of a system that ensures its smooth running. You can be fine with the bare minimum which is really all you need but then you will be compromising on the quality a bit. The supplied power brick is determined by the expected highest level of power that your PC demands. So when you choose to go for a lower supply capacity of power, you might face a lot of consequences which include insufficient supply. And your pc definitely needs more than that. So for example Apple switches the power automatically, in this manner the power supply starts to die out, the internal battery of the pc would act as its replacement. But once the internal battery dies out, as the GPU and CPU use increases, it might lead to a system wide crash.
Long term use of the power supply in this way means that you're regularly trying to push it past its limits. For a well designed supply, it'll be current limited, so it won't try to operate past safe current and heat limits, the main issue will be lower power efficiency at those loads than for a properly sized supply. However, a not so properly designed supply will suffer thermal stress with repeated overloading. It will not last as long as the full spec supply.
But all this comes with the assumption that the pc is going to work with the least amount of supply too. If there is a proper communications channel in your PC, it may refuse to work with the unauthorized supply of power. For example if a person runs their laptop on 80 watt adapter for some time while it called for a 120 Watt adapter, the battery would warm up and then that’s it. It would perform as it normally does but it would not be able to handle the games on the laptop as they take on too much performance power.
If there is any CPU intensive task, you can plug on the battery and if the battery is removed in the process it would take a lot to turn it off, So while the CPU intensive tasks lie outside of the gaming it would still shut the laptop. There are a lot of tests that are done with adapters of different capacities. It is not even harmful as the laptop will not explode. Neither is the power adapter. So what really happens if you use a lower wattage power adapter? Nothing really. It all depends from pc to pc.