You probably heard about overclocking. Global corporations and individuals alike, all around the globe push the hardware beyond its default specifications.
Factory-overclocked graphics cards, factory-overclocked mobile processors in gaming phones, the custom-overclocked desktop processor at your friend's house - overclocking is everywhere.
And sure, why not? Everyone wants the best performance. But why on earth would anyone want to decrease the performance of their hardware?
The main reason is reducing power consumption. Underclocking is usually combined with undervolting. As a result, the processor (or graphics card) consumes less power (improving battery life) and outputs less heat.
But if we are so concerned about battery life and heat, why not simply use the weaker chip? After all, GTX 1060 consumes less power and outputs less heat than GTX 1080.
In most cases, the higher-tier components even after undervolting and underclocking are still significantly more powerful than the lower-tier components.
The same goes for laptop-grade and desktop-grade hardware. Differences between them are better described in this article https://steemit.com/hardware/@hwtrendsetter/your-laptop-may-not-be-as-fast-as-you-think
Generally speaking desktop components are significantly more powerful (at the expense of higher power consumption and heat output). However underclocked desktop-grade components (when put in a laptop) still outperform overclocked laptop-grade components.
So if the customers want the best of both worlds - great battery life and great performance (and no problems with heat), underclocking is the way to go for the manufacturer.
Another reason may be marketing. iPhone SE 2020 comes with the most powerful smartphone processor on the market* (*or at least according to that one benchmark that favors Apple. And we run that benchmark when that processor was in a different iPhone, that obviously comes with better cooling and much better battery).
When underclocked, that processor is not any faster than the previous generation Apple chips. In order for the battery to achieve reasonable runtimes, that "most powerful processor in the world" had to be underclocked. But for the customers it doesn't matter - they are still convinced that they are getting "the best".
And to be clear - Apple is not the only one doing that. Many Android manufacturers (including Xiaomi) were engaged in similar practices at some point in time.
Finally, individuals may be underclocking some components in their desktops.
If someone got for Christmas (or won in a competition, or bought during an episode of compulsive buying disorder) a new, powerful graphics card, put it in the system and it works. But their power supply or cooling system (or both) are not up for the challenge and the card overheats or even shuts down the computer. In that case underclocking and undervolting allows such individual to use the graphics card (which even when underclocked will be still far more powerful than his old gear) before he upgrades the rest of his components.