For me, probably they won't do a lot useful - I rarely leave the Colonia region, and I have one ship that I use for everything, so I'm really not the target audience for themI find it hard to imagine that the fleet carriers will provide any more gameplay than what exists already.
I think the convenience of being able to move lots of ships at once instantly within the bubble will be fairly popular. 500LY is basically anywhere-to-anywhere within the bubble, and puts the Pleiades in reach of most of it as well. You can do most of it already with FSDs and ship transfers, but being able to fuel up the carrier well in advance, and then just jump straight into the action when the time comes, is probably a better balance for keeping the gameplay feeling active.
I'd expect a network of deep space ones to gradually form for exploration repairs - the stations mostly stop once about 7k from the bubble, except on the line to Colonia, so there's a lot of the galaxy where this would be useful.
The extended per-jump range is going to - perhaps slowly - break some previous exploration records, and maybe give rise to some new types of expeditions, even though they'll probably be much slower per-hour than even a mid-range ship in much of the galaxy.
People could use them to set up "taxi" services to distant places (bubble-Colonia, for example) - in a reasonably balanced way.
Once the concept of player-driven shuttle services and deep space networks is set up - and actually working in practice, balance issues worked out over a few releases, etc. - rather than just a "theoretically this could happen" - that then allows the possibility for Frontier to add more deep space content in a future release which is either outright dangerous or just requires ships specialised for something other than exploration, without it being really hard work to get the required ships out there in the first place. That won't be there on release, but could be added later.