Mostly so I can check my emails on the move.
Also, if I do end up leaving this place, I'll likely not be carrying around my laptop so much. It works for me here, because of the whole living where I work deal, but chances are whatever job I end up getting next, that won't be the arrangement. The new laptop is kinda heavy...
I've read more books on my phone in the past year than I have read physical books in at least the past 10 years combined.
But everyone jokes about how tiny I keep the font on my work computers*, so your mileage may vary.
*Joke's on them because I can easily read everything on their screen when I walk by, but when people try to shoulder surf me they just squint while I casually close inappropriate tabs like a goddamn champion.