If you already own Fallout 76 on Bethesda.net, you'll be able to get the game for free on Steam too, but only for a limited timeframe.
To take advantage of the offer, you'll need to link your Steam and Bethesda accounts together before midnight ET on the 12th of April. If you do take them up on the offer though, atoms an Fallout 1st subscription's won't come with you, as these remain exclusive to the platform they're purchased on.
Items purchased to use in the game will remain across both platforms, though, as well as general progress within the game.
If you take advantage of this offer, or purchase the game on Steam, you'll also get a copy of Fallout Classic Collection, which has the first two titles and Fallout Tactics included, so it's worth taking advantage of this timeframe for those games if you don't already own them.
The two platforms will be crossplay compatible, so you won't need to do this if you want to play with other players on the Steam platform, but if you prefer to be in the Steam ecosystem, or simply want to pick up some classic Fallout titles, it's worth doing.
Fallout 76 releases on Steam on the 14th of April.