The best part about bib overalls is that there is no belt or waist band. Climbing on and off a tractor or bending over in the garden is much easier without something pinching your mid section. Also, sawdust, wood chips and bugs tend to fall on through without much delay -- last week a chigger got under my shirt and worked his way around my beltline until I was well bit. But overalls would have let him pass through.
Plus for really hard work, they offer a layer of armor -- that bib offers protection when carrying firewood, lumber, tools, et al.
I have a variety for different occasions -- lightweight for summer, lined for winter, white for special occasion's, and even some made from a heavy Scottish wool just for Thanksgiving at the cabin. I like Dickies and Wear Guard much better than Carhartt for lots of reasons [fit, cut, buttons, length of zippers, lining, etc.] and they are much cheaper. Plus those engineer pin stripes make a real fashion statement for the man outstanding in his field -- pun intended.