Why do we write dedications in books?

When my siblings and I were clearing out my dad’s bookshelves (he died earlier this year), I made sure to keep any books in which I’d written a personal dedication to him. For some reason I baulked at the idea of them passing into the hands of strangers, or just being left to languish in the anonymous corners of charity bookshops. Worse than that would have been copies of my own novels, dedicated on the title page to ‘Dad’. (‘So even his own father didn’t bother keeping them…’)

Why do we write dedications in books? I understand it as a romantic gesture: a way to show off your tremendous good taste to someone you fancy. But as for ordinary occasional presents, the custom is slightly mysterious; it’s not as if many of us wrote messages on CDs or, earlier, LPs. I tended to include dedications in books more often in my younger years.