You can hold a tablet easily in one hand while using a wrench in the other. That's a little awkward with a 12" laptop.
Honestly for me, PDFs are far more readable on a tablet than a laptop. The tablet is just plain easier to hold, and text appears clearer on even my non retina iPad mini than it does on any laptop screen. I can set the tablet down darn near anywhere, in spots the laptop won't necessarily fit. I can slide it into a waterproof sleeve or even a gallon size zip top bag to protect it from splashes of anything liquid (works well in the kitchen or when painting/crafting, so I'd imagine that'll do fine for the garage too). And of course, as eschwartz noted, it's more portable overall, especially since your Dell's battery is dead.
But I've been using iPads for darn near everything for a few years now; my laptop sees very little use. So perhaps I'm a wee bit biased.