I use them for whatever task that comes to hand that they're capable of performing well. Hunting/hiking/box opening/cooking/nail trimming/juggling (but only with some burning torches in the mix).
I use thin knives with very hard tempers/abrasion resistant steels for slicing fragile (tomato) or abrasive (cardboard) materials. I use thick knives with less abrasion-resistant steels that have much higher levels of ductility and shock resistance for heavier work, such as splitting, batoning, scraping and some chopping. Seeing as how I've never once needed to build a log cabin in the wild, the notion of lugging an axe along with me has always seemed a tad stupid. I know many people here who would have no hesitancy at all in using an Estwing hatchet to chop wood; and yet---when I point out to them (with comparative pictures no less) that the blade geometry of the Estwing is actually no more robust than a heavy fixed blade and then go on to mention that the Estwing's steel is greatly inferior in both its edge holding ability and resistance to damage when compared to many of the best hard use blade steels: 4140 in the low 50s HRC versus 5160, L6, INFI, 3V, etc. in the high 50s or even low 60s---they balk and then usually get rather condescending in a "You just don't understand" kind of way. GB hatchets are even finer geometry, and will suffer greatly from a slightly glancing blow into hard wood, or hitting a hidden hard object like a nail or small stone that's imbedded in the wood. That doesn't make them a poor tool, just one you have to use good sense with, like anything else. I have knives that can take far more abuse than most hatchets, and hatchets that are sharper than most can get their knives.
Batoning properly is a skill, just as dicing vegetables really close to you fingers is a skill---you can do it right and also really, really wrong. People are certainly entitled to their opinions but the pure, blanket "Knives are for cutting" statement has always struck me as rather obstinately unconsidered. It essentially suggests that a 1/16" Swiss Army Knife blade and a 5/16" bolo blade are intended for the same scope of work.
Knife suitability is based upon geometry (both blade and edge), steel selection and heat treatment. A scalpel performing facial reconstruction is on one end, and a beef splitter lopping through huge chunks of bone and meat is on the other. Both of them are for *cutting*.