Steam activation is needed because the game has been hugely updated in the time period after it was burnt on discs and before it will be released on 30.6.
When you install the game, you need Steam to get all the patches. After that, you can just delete Steam forever from your computer* and play D:OS completely without Steam. The game is DRM free, so you can burn it on your own disc, transfer it to another computer and so on. If you want to avoid Steam completely, you might consider buying the digital game either from GOG or directly from Larian Vault.
*That is: some future updates or expansions (which are really just expansions and are not required to play D:OS as it is) might be handled through Steam as well - but I think the patches will be eventually available on Larian Vault.
