When Veldora disappeared, it was said to have a large impact on the surrounding lands. Why would it make such a large difference if he was already imprisoned and unable to use his skills?
Typically in any sort of media that uses some sort of superpower, the characters have some sort of aura. This can be seen initially from Rimuru unknowingly exuding a powerful aura. I believe it was said in the LN that Veldora isn't able to control his aura that well, so regardless of what he's doing his aura is gonna be present.
Your question isn't quite clear to me since your introductory clause doesn't seem to be relevant to what you're asking. So I'm gonna interpret it as: "Why does Veldora still have a large influence even if he is imprisoned?"
Edit: I think I understood the question halfway through writing my answer.
Not edit: VV
Well first of all, assuming the seal initially blocks his aura or even suppresses it, the forest will have a large area that is no longer covered by Veldora's aura, meaning weaker life-forms can now habit the area that it once covered. The disappearance of something so great is bound to have a large effect. Later on, assuming aura/magic propagates through the seal/air, if Veldora stays imprisoned for so long, the area around him will be saturated with magic power from his aura. This can be seen when Rimuru is exploring the cave when he arrives and there is an abundance of rarer materials. Since the surrounding area is full of magic power from Veldora's aura, then if magic follows similar laws as matter does, it will keep spreading to try to keep an equilibrium of magic particles and eventually reach outside the cave, eventually covering another area and preventing weaker life-forms from living there again. The fact that he is still there causes a large influence, even if he is sealed up. When Rimuru devours him, his aura is no longer present, causing everyone that is capable of detecting that his aura isn't there anymore to know. Who wouldn't be nervous if one of the strongest life-forms in your world just disappeared all of a sudden?