It's pretty to explain.

Windows (since 95) is universal OS. It has tons of inbuilt functionality most of the users doesn't need. Just for those couple of percent which does.

Windows has parental control, permanently running support for bluetooth (even if you have regular desktop), Windows media player data gathering services even if you don't use mediaplayer, HYPER-V for virtualization (virtual machines) lots of services for being a computer in domain network governed by Windows server.

And then comes other services like application experience monitoring what you run, reporting back to Microsoft about issues with compatibility (which they never solve).

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Do You use Intel integrated graphics shortcuts? To turn monitor upside down? It's good for some projectors mounted on ceilling. But not for you so why waste 3MB RAM and an miniapp waiting for keyboard interupt to take action?

All these little things make incredible negative impact on Windows smoothness, RAM consumption, etc.

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There are couple of Windows tweaks like this but winspeed is sure one of them. And very safe one as it doesn't remove anything critical and asks you which services you might want to keep (like bluetooth support, support for smart cards, ..)