Please explain if I need Direct TV for my Smart TV?

You'll lose authorization for those streaming channels if you cancel your DirecTV service.

As noted, you need paid tv service from someone to watch those channels but it doesn’t have to be Directv. If that is all you watch and you haven’t even activated your Directv receiver, you should consider some other tv service. You can probably find something cheaper than Directv. Just be aware, if you are a new Directv customer, you are likely on a 2 year contract and whatever price you are paying for Directv is probably only good for a year.

DirecTV is service by satellite. Some individual channels will allow streaming (smart TV, devices like Roku, Firestick, etc.) with your DirecTV service.

If you cancel DirecTV, then you lose all associated perks. So you could not use those channel apps, unless they give any free content themselves.