My opinion: There are a lot of "tactical" blades, tomahawks included, out there that are a form of fantasy blade. The fantasy is just modern military instead of wizard-slaying barbarian.
Then there are the ones that are actually built to serve military and law enforcement needs and might be specialized in a direction that doesn't necessarily make them the best woods tool. They may be great in the woods, but that's not their primary intended usage. Even fighting is not the primary intended use for many of them (although some, particularly the ones based on Filipino headhunter axes with the forward spike, are primarily weapons), though that is usually at least part of the design considerations.
As to why someone who is non-military or LEO might want one, why not? There are a lot of people who like to have a little bit of the trappings of warriors even if they themselves are not. Look at all the MOLLE gear and ARs at the next gun show you attend. Heck, I make no pretenses to badassery, but I have a mil-surp canteen that I take water out to my shop in, and I have a fondness for olive drab. Might be a little silly, but what harm does it do?
Separating out the fantasy from the tools carried into harm's way, a tactical 'hawk is a compact multitool that should increase the tasks a soldier can tackle. Shaw11b has given good examples of the tasks a good 'hawk might be called on to perform in a warzone. All of those are tasks that would be hard to perform with a rifle or bayonet. In the small chance that a solider is forced into using sharp steel to actually fight with (which does happen upon occasion, even in the age of drone airstrikes), a 'hawk affords advantages of reach, power, and hooking/trapping/smashing/blocking/redirecting abilities that a knife generally does not.
A lot of folks buying tactical 'hawks are civilians who may never press them into service the way a warfighter might. Does it matter? Any difference between that and carrying a MOLLE pack and a Kabar out into the woods? Plinking with an AR? They help support the tactical 'hawk market, meaning that there are a growing number of 'hawks out on the market that might serve a warfighter in a tight jam. It's just up to the warfighter to sort through what's available and pick out what will serve his needs, just like any other piece of non-issued equipment he carries.