Hugely instrumental in my career. Landed me my first ever job (2008) and has been responsible for underpinning most of my consultancy career (2013–now): https://csswizardry.com/

As long as one sticks with it, I cannot overstate the power of a good personal website.

> Also I can't decide what to write about, and whether to make it more nerdy or more professional.

As with most decisions, just make one. You can always change your mind later.