[imrejonk]

And: they have a crash cart (keyboard, mouse and display) and battery backup built-in. An old laptop is perfect for starting a homelab. The only major downside I can think of, and as another commenter already mentioned, is the limited storage (RAID) options.

[HPsquared]

A lot of older 17" laptops had dual HDD slots.

[kassner]

Or DVD drives in which you could add a disk caddy.