Sol here. So after seeing the subs for the beginning of Ultraman Geed, I know some stuff about the Ultra Capsules, but somethings don't make sense. Hence the 'theory' part of this blog. Let us begin. (Don't worry it's going to be short this time)
So we know Ultraman Hikari made them in response to the conflict that Belial caused. They are weapons, but the question is...how are they useful. Well, what do we know.
From the trailer for episode 2 we can surmise that the Capsules now reside inside human in energy form, giving these people super powers...And I think that explains it all. The Ultra capsules were meant to be used by Ultra, probably the average warriors, temporary power boosts, the strength of the elites for short periods of time. It has been stated that the Ultra Brothers are the elite of the Space Garrison now.
Even before that I had a suspicion/theory on how they would work. That was that...they were based on the Fusion Cards of Orb, and were basically a tempt at mass producing the greatest weapon of the Ultras...no not human hope...Fusion. Fusion tends to create Super Ultras, and they are defined as Ultras bearing the power of multiple Ultras. Now depending on how you define that, that means anyone from Gaia, after his version up, to Ultraman King and Noa, whose power is several times the magnitude of normal Ultras.
So then we have a system of giving Ultras the powers of other Ultras, often times stronger Ultras, to use in battle. They are basically, in universe, another form of the Fusion Cards Orb uses. Therefore, the Ultra Capsules are in fact, weapons left over from a war.